Many people spend thousands of dollars on custom made wood speaker boxes. Most of these people do not know that you can make your own custom speaker enclosure for cheap.
First off, you need to draw out a sample of what you are looking to make. Get the demensions of your trunk to make sure that your box will fit correctly. Next you can use your measurements and samples to figure out about how much fiberglass and matting you need to make your box.
We will just assume that we are going to make a one 12 inch subwoofer box that fits into the rear trunk panel. Make sure that you look at the specs on the subwoofer to make sure that the volume of the box will measure out correctly.
Next you can get your fiberglass and matting. Any auto parts store should have what you need. You need to get about a gallon of fiberglass resin and about 5-6 packs of fiberglass mat. Dont forget to make sure that the fiberglass resin comes with hardner.
Now we can set up how we what our box to fit in. Lay down painters tape in your trunk where you want the box to fit into. Make sure you go a little farther than you will actually make it so you don't get any resin on your carpet. Go ahead and mix your resin with your hardner and use a paint brush to paint on the resin over the tape to create the back of the subwoofer box. Cut out your fiberglass mat and lay it down over the resin. Paint another layer of resin over the newly laid down fiberglass mat.
Now you just wait a couple hours until the fiberglass has hardened. Pull out the back of the enclosure you have just made. It should fit snuggly into the trunk panel. Now you need to add 3-4 more layers of fiberglass matting to the inside of the back of the enclosure. This is to reinforce it, so it doesn't crack when the subs hit.
Now that you have made the back of your subwoofer enclosure, we can start to make the front where the sub will go. For this part, you will need some 1/2 inch mdf, some dowel rods, glue, and some panty hose. Lay your sub face down on the mdf and draw a line around it. Cut out the circle with a jigsaw. Now you need to measure the inside of the sub ring, so you will make a ring that the subwoofer will fit into. Usually, subwoofers come with a measure ring you can use to make this.
Now you can position the ring however you want the subwoofer to go. Cut your dowel rods so they keep the ring in place, where you want it to be. Glue the dowel rods to the inside of the back of the sub enclosure that fits into your trunk panel. Glue the other end to the ring. Now you should have a structure of what your sub box should look like.
Now grab the panty hose and stretch them over the ring to the outside of the back of the box. This should make the box become enclosed. Staple or glue the panties hose so it stays in place. Paint a layer of resin over the panty hose and let it dry. You should now have a subwoofer enclosure that fits into your rear trunk panel. Now you need to cut out the inside ring where the subwoofer will go. Layer the inside of the enclosure with about 3-4 more layers of fiberglass mat and resin.
Your sub box should be complete. Now you can drill some small holes in the back of the enclosure for your speaker wires. You can also grab some carpet (color of your choice) and lay the carpet over the front of the box, so it looks like it was suppose to be in your trunk.
This is how to make a custom subwoofer enclosure. You can also lay bondo down over the fiberglass and sand it to down to paint it.