This project was my first fiberglass experiment, so it took longer than normal. I think I spent about 1 week from start to finish, and overdid it a little. However, when I did finish, It looked good, (I just needed to spend a few more hours filling in pinholes with bondo to make it perfect), It was incredibly strong, and I sounded 10x better than the old MDF box... plus it saved a ton of room in my trunk. Pics are at the bottom of the page.
Fiberglass resin and hardener
Fiberglass cloth and/or mat
2x4'of 3/4" MDF
a palm and/or angle sander
disposable paint brushes
respirator mask (recommended)
laquer thinner (for cleanup)
an old shirt or similar cotton cloth (with no holes)
The first step is to tape off (with tape and newspaper/aluminum foil) the area that you want to fiberglass. This is an important step because if you don't tape off a large enough section, you might get fiberglass resin on something unintended. If that happens, good luck getting it off. Also, larger is better..you can always make your rough casting a bit smaller after it dries (using a jig saw). Next, lay the fiberglass matt or cloth onto the area. then mix your resin (mine was 14 drops hardener to 1 oz of resin.. but read your label) and apply with a cheap paintbrush. Push the paintbrush in and brush it on until the entire piece of fiberglass mat turns from white to the same color as the resin. Don't waste time once you mix your resin, because you only have about 10 minutes of working time before it becomes like jelly and begins to harden into fiberglass. Allow to harden before adding more coats. It will probably require about 3 coats or so to make it rigid... although, you can use as many as you want. You will notice after the 2nd coat that it is becoming quite hard and sturdy. After 3 coats, I could jump up and down on it without feeling any flex at all.
Once you get the basic casting complete, draw a line around the top edge of your casting, about an inch or so from the top. Use a fast cutting saw (such as a jigsaw), to cut the jagged edge off of your casting.
Next cut your MDF rings to fit. The first is your baffle or mounting ring. The inside of this hole should be just big enough for your sub to mount into (the sub will screw into this ring), and it should be wide enough to where there is roughly 1.5 inches beyond the outside diameter of the subwoofer (mine was approx. 3in. wide, with a center hole of about 11in. for my 12in. subwoofer, so the total outside diameter of mine was about 14in.). Next cut your top ring. This ring will be used to adhere the fiberglass to. The inside diameter (hole) of this ring should be just slightly larger than the outside diameter of subwoofer, in other words, this ring should be able to fit over your sub without touching it. It should be about 2in. wide and the outside diameter should be the same as the first ring, such that, if you put the second ring on top of the first, they will be exactly the same size on the outside, and the second ring (top ring) will allow you to see about 1.5in. (all the way around) of the inside of the first ring. Now, screw the 2 rings together, the smaller of the 2 on top.
Next use wooden dowels to hold this ring where you want it. Cut the dowels to length (probably use 4 sticks), hot glue them to the inside of the casting, and the other ends to the back of the first MDF ring (keep in mind that the the rings should be far enough above the casting, that the sub will be able to fit into the enclosure without hitting the bottom, this will require you to measure the depth of the sub being used).
Next use an old cotton shirt or other thin absorbant cloth to cover your casting. Wrap the shirt or cloth around the outside of the casting and cover the MDF ring such that the entire open end of your casting is covered. Pull the slack and wrinkles out and pull it as tight as possible. I used duct tape to temporarily hold the shirt to the outsides of the casting while the resin dried. Now apply a layer of resin on top of the shirt, making sure to especially soak the area where the shirt touches the casting and where it touches the MDF ring (try not to apply the resin all over the center of the sub hole... as you will be cutting it out). Wait for it to dry and apply some fiberglass cloth with more resin. When the cloth is rigid enough that it does not move, cut the center of the sub hole out with a jig saw or dremel tool to make it flush with the inside edge of the top MDF ring.
Now have at it with your fiberglass, I recommend going at it from the inside of the box, that way the outside is easier to sand down. Apply how ever many coats you want of resin and fiberglass mat or cloth to the inside of the shirt. Try to get as much as you can in the cracks around the MDF rings and where the shirt meets your casting (this will make you enclosure incredibly rigid when you are done). I used 3 coats, and it was plenty... go until you feel it is enough.
Now, you are pretty much done fabricating, just apply as many more coats of resin and fiberglass cloth or mat as you wish (I would do it to the inside of the enclosure for sanding purposes), just make sure that when you are done, that you can stand on your enclosure and it will not bend, at least that is what I did... mine is stong enough for 2 guys to stand on without the slightest flex. Once you are satisfied with the strength, it is time to sand.
I used a 120v Ryobi angle-grinder ($29 from home depot) with a sanding attatchment to do the initial sanding... it is a LOT of sanding. It will take you a while to get it perfect, but keep going at it. If you encounter any shallow spots where you see that the fiberglass is getting thin, re-apply to the inside (a coat or 2) and keep sanding. Once you get all the major bumps out, switch to a palm sander with some finer grit to smooth it out. If neccessary, use Bondo to fill in the pinholes that you might notice when painted. After all sanding, just prime and paint your favorite color (or you might cover with vinyl or whatever you want).
Now, just drill a hole just big enough for the sub wire, run it through and dab some silicone to seal it, connect the wires to your sub, and secure it to the MDF rings with screws.
These are of the first step, taping off the section to be fiberglassed using masking tape and newspaper.
These show how the wooden dowels should hold the MDF rings, and how the MDF rings are supposed to be constructed and put together (pay close attention to the setup of the MDF rings).
These show the cloth wrapped around the casting and over the MDF rings, pics were taken after a few coats of resin and the sub hole was cut out.
These are of the finished product installed in the car.