Building oddly shaped speaker enclosures can be very easy if you take the right steps. In this video you'll see examples of how we build our fiberglass enclosures.

Things you'll need.
1. Chopped fiberglass mat Link to buy 3M/BONDO 488 FIBERGLASS MAT
2. Fiberglass resign Link to buy Evercoat 498 Fiberglass Resin
3. Cheap paint brushes
4. Rubber gloves
5. Aluminum foil
6. Masking tape
7. Cardboard
8. Spray glue Link to buy 3M Spray Trim Adhesive
9. Scissors
10. MDF wood
11. Jigsaw Link to buy my favorite Jigsaw

First you'll need to take a rough measurement of the area you are going to be building your box. If the airspace is there, then go ahead and mask the area off with newspaper and masking tape to ensure that you don't get resign on the vehicle's interior. Now you can begin to lightly spray glue the area and line it with aluminum foil. Once you do that cut the fiberglass mat up into various sizes pieces. Some big, and some small. Mix the resign according to the instructions on the container. Apply a thin coat of resign to the foil. Lay the fiberglass mat into the area as you apply generous amount of resign. When it dries apply a second layer.
 
You are going to want the box to be very rigid so that it doesn't flex while the subwoofer is playing. once enough layers have been applied you can remove the box. I'll usually mark a line around the edge of the fiberglass indicating where i want to cut it. Cut around the edge of the fiberglass pan that you just made. Lay it onto a piece of 3/4 inch MDF and trace it. this piece will be your woofer face. If you don't want the face of the enclosure to be flat, you could mount a wooden ring in place and stretch grill cloth over the ring, attaching it to the enclosure. Apply resign over the grill cloth to harden it, then lay fiberglass over it to make it rigid. If you do decide to make the front flat, then you would simply fiberglass your MDF plate into place. Grind it down and Wrap it!!!