Customer’s Fiberglass 10″ Sub Enclosure For Subaru STi – How To

Posted: January 21, 2013 in Custom 12-Volt Installations, Toby Broadfield's Work
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I typically don’t take build pics of projects as basic as this. But I figured it would make for some good write-up material since this type of enclosure can be used in virtually any vehicle. The goal is that if I can just show some detailed step-by-step instructions, I think it will convince a lot DIYers that they can accomplish this on their own. The most common size of driver that is going to work in an area like this is a 10″.

First off the area I’m working with:


Once you pick your area, use quality masking tape to cover the entire area and go past the actual area you are going to lay fiberglass mat. It’s important to keep the tape as flat as possible and not to kink up on you. It’s obviously not possible to make it perfect, but do your best:


Next, and actually a really important step, is to use a marker to mark out the shape you want the back of the enclosure to be. Now there is no rule to where you make your mark, but the following picture should give you an idea of why I made it where I made it:


As you can see in the picture above, I followed the natural contour along the top of the OEM panel. Then dropped the line down towards the front of the trunk where I want the enclosure to stop. After that I brought the line out from the panel towards the back of the trunk. When doing fiberglass like this, the shape of the back plate and the shape of the area where the sub mounts 100% dictates the final shape of the enclosure. Once you see some more steps you will be able to come back and see why I drew the line like I did.

Next is to use some plastic to cover the rest of the trunk. You can simply tape it to the blue tape you laid down earlier. Obviously make sure you do not tape on the inside of the line. It’s very important that the marker line does not get covered up. Next you will use some carnuba wax to wax the masking tape. Do several coats. You can also use a PVA mold release, but pretty much every car guy has some wax lying around… so that it makes for an easy option. Make sure you go right up to the line, but not over it. The goal here is that the line will transfer to the fiberglass.

Below is a layer of fiberglass mat that is roughly cut to fit the area. You obviously want to make sure it extends past your line a little. This will be more obvious after you brush on some resin. Note: this piece of mat does not need to be one piece. You can piece it together with pieces overlapping a little. Once you apply the resin and it dries, it will be one solid piece.


Now you will need to apply some resin to the mat. Mix according to the manufacturer’s instructions. Once you get use to doing this type of thing, you can add more or less hardener to speed up or slow down the curing process. Speeding up the process is good since you will need to wait for each layer of mat/resin to dry before you lay the proceeding layer… and nobody likes to watch resin dry. However, if you mix in too much hardener, the batch of resin may set up before you get all of the mat coated with resin… thus wasting resin. You need to really flood the mat with resin. Use a disposable 2″ brush found at Home Depot etc… they are about a $1 each. Try to make sure there are no air bubbles. You can use the end of the brush to “stipple” the mat as you apply the resin. This will help force out the air bubbles. Don’t worry, you are going to have some tiny ones… it’s no big deal. It’s not like we are making cosmetic carbon fiber panels here. Any air bubble that is a 1/4″ diameter or bigger you will want to remove though. Once the mat is coated with resin, making sure to go past the marker line a little, then you can let it dry. Resin applied below:


Once the first layer dries, you are going to repeat the process with another layer of mat. First check to see that there are no nubs sticking out though. Sometimes if a sliver of mat doesn’t lay all of the way down when applying the resin, it will harden and stick out. This can interfere with the next layer of mat… so quickly knock it down with an air sander or similar. Then simply lay another layer of mat just like the first. In the following pic there are 3-4 layers of mat:


Keep continuing the process until you have about 6-8 layers of mat… you want around 1/4″ – 3/8″ of thickness depending on how big the area is. It will also depend on what weight of mat you use. The stuff I typically use would be the weight of what you would find at the hardware/automotive type stores. The following pic is around 7-8 layers of mat:


Once that last layer is fully cured, pop that bad-boy out of the car:


As you can see, the marker transferred/bled into the first layer of fiberlass. So now you simply cut on the line to trim it down to size. I like to use an air saw for this. Note, I actually forgot to apply wax before I laid the first layer of mat. So as you can see some of the blue tape stuck to the back of the plate. Masking tape has kind of a wax layer on the non sticky side, but it will still stick to the fiberglass… so don’t forget the wax or mold release!



Now you can remove the masking tape and plastic from the car. Time for a test fit:


Now that you have the main back plate, it’s time to figure out what shape you want the front of the box. Typically you will just make a round ring that will hold the sub. It will give the front of the enclosure a natural flowing look as you will see later. I usually like to flush the sub(s) to give it a cleaner/finished look. It also gives you the ability to do a custom flush grill if you choose. I make the sub ring out of 3/4″ MDF. I also made a second really slim ring to mount on top of the first… this is what will create the countersunk area to give the sub the flush finish. Then simply make some legs to hold/suspend the ring where you want it. This step is VERY important as it will determine the final shape of the enclosure.

Here is the ring mounted up. You can simply screw into the legs from the backside to hold it in place. When angling the ring like this, you will need to test fit and trim the legs several times to get it to the angle you want:


Out of the car so you can see it a little better:


I then mixed up some resin and finely chopped fiberglass mat to help bond the legs to the back plate:


Back in the car to make sure nothing warped out of place:


Now it’s time for the best part… stretching material over the whole thing. This is what will give you final shape of the enclosure. I like to use fleece or speaker enclosure carpet. They are both kind of thick, yet stretch well. I used a thick fleece in this example. You will lay a piece over the front large enough to cover the whole project by about 12″ on all sides. You will glue it with some “good” spray adhesive to the backside of one end of the enclosure. You will then stretch it to the other side and glue it in place. You will work all the way around the enclosure stretching and gluing as you go. The tighter the stretch the better the fleece will hold its shape when you apply the resin. The following pic is with it all stretched and glued to the back:


Next you can trim off the excess material with a razor blade:


This what it looks like from the front side:


Another test fit:


Next you will resin the fleece. Make sure you soak the shit out of it anywhere it touches the wood ring or the back plate. You want to make sure it soaks through and bonds really well in those areas. Note: you obviously don’t need to resin where the sub goes since you will be cutting that out after the resin dries. So just resin slightly to the inside of the ring to make sure it bonds well with the wood. After that dries do another coat:


After the second coat dries cut out the area where the sub sits. I rough cut it with a knife first. Then use a 1.5″ sanding drum on a die grinder to get it perfectly up to the ring:


Turn the enclosure over and trim the remaining material off the backside. I use an air sander with a 24 – 36 grit sanding disc. Finished results:


Now you will need to strengthen the front of the enclosure. You will need to add several layers of fiberglass mat just like you did on the back plate. Lay mat right up to all of the edges of the enclosure and to the edge where the sub sits. Don’t worry if any hangs over… it’s easy to sand that back off with the same 24 – 36 grit sanding disc once it’s all dry. If you are simply carpeting it then you can sand it down with 36, 50, then finish with 80 grit. This is after 6 layers of mat and finished up to 80 grit:


Time for carpet! Use a quality spray adhesive… 3M is not one of those 🙂 Even better is to use some contact adhesive out of an automotive spray gun. Finished results:


Carpet trimmed on the back side:


In order to mount the enclosure, you will need to place a piece of wood behind the OEM trunk panel and hold it in place with a couple of screws through the front of the trunk panel. You can then drop the enclosure in place and mount it by running some screws through the back plate of the enclosure and into the wood you just mounted. In the picture below you can see a couple black phillips screws that go through the back of the enclosure:


Once you are happy with the way it fits, take it back out and add some speaker wire to the enclosure. Drill a hole in the bottom and run the speaker wire through and then seal it up with some hot glue or silicon etc. Remount the enclosure and then add the sub of your choice:


  1. bigfan says:

    Awesome. I’ve been waiting a long time to see more posts!

    • Broadfield says:

      Yeah, winter time is the busiest time of year for me at work. Then factor in all of the holidays… I don’t get much time to myself or time to post on here. The blog will pick back up as usual though come spring. Thanks for the support man!

  2. racer4life77 says:


  3. Nsxctasy says:

    You said 3M is not good spray adhesive. What kind of Adhesive do you recommend to hold the fleece while you stretch it over the enclosure & to hold the carpet at the end?

  4. A Tang says:

    What methods do you use to mount the enclosures to the trunk so that they don’t slide around? I get real worried about drilling into my trunk. Are there any other ways to mount it without the mounting points showing out in the open?

    • Broadfield says:

      Mount it just as I described in the post: use a piece of wood that you put behind the side panel carpet and screw through the carpet and into the wood. The screws are not screwing into the car, just the wood. The wood will then give you a mounting point for the box.

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