you will need some strong materials to keep shape of what youre trying to build, I like to use MDF as the skeleton. For example, if you were to make a mold of your spare tire well as your enclosure, you will need the Following. Fiberglass and resin, sandpaper, paint brushse, Bondo, Foil, the fat (2"?)roll of masking tape, mold release wax, a sheet of MDF, jigsaw, air saw, cardboard, some thin fleece, some rags, some masks and a small handful of other tools.
First pull apart your trunk and pull all the panels where youre not gonna step on them. Then lay the foil in your trunk well very liberally, but try and keep it flat and tape it in place. Next put on the wax. Then Slather on the Fiber glass and let it dry. Then take it out and mark off any excess (around the edges) that you dont want to be there in the finished product with a black sharpie, and use your sawz all to cut all that excess off. Cut some rings the diameter of the speakers you are mounting. Then Cut some braces of different lengths for how you want your subs to angle. Remember to cut them with equal angles. Then Cut a ring the Diameter of the spare tire well, and make some more braces for the height of the hole. Use some Hot glue and glue about 8 or 10 of them around the diameter of the bottom of the hole. Glue the ring on, then follow suit with the5 or six braces for the Height of where you want the subs, and how you want them to sit. Glue the rings on and give them a good once over, cuz thats the directin your subs will fire.
Next get your stapler out (heavy duty) and starting at the top, stretch your fleeces tightly over the box, and staple at different increments, try and keep the fleece tight, and you wont have any wierd ripples. Do it. All the way to the bottom. Then Slather a crap load of fiberglass in the fleece. the more the merrier generally. Wait for it to dry. Use a DA sander with like a 40 grit sand paper and take off the big chunks on the outside and make it semi smooth, perfection here dont matter yet. Next get out the glass mat and on a board, coat some 8x8 strips with the glass and put them in different places on the box and slather those up with more glass. Let it dry. Now you have the general shape of your box. Cut out the holes where your speakers go with your sawz all. Then, on the whole box, go over it again with 40 grit real quick, throw some bondo on, then switch to 120, and fill any big gaps with bondo. then depending on if you wanna paint it or not, hit it with more bondo and super high grit (400-800-1200-1800 grit)sandpaper, then more bondo, and such. When painted, like a car, any defect will stand out. And krylon sucks. If youre gonna paint it, you want it to look nice. So spend a hundred bucks and have a pro paint it. you dont wanna show it to your friends and say hey! look what I built! And then you suddenly become the anal end of many jokes. IF you dont paint it, get some contact cement, and choose a covering. Carpet is easy, vinyl is harder. Either way you go, stretch it along the SIDES, and stretch it good. and use lots of contact cement. Cut it with a razor blade, wire up your subs, and spray some super 77 glue inside, and put a little poly-fill inside the box, but just enough to cover the walls about an inch thick sheets, so you can eliminate some of the standing waves. Mount your subs in the box and call it an afternoon (or 40, if youre building it between customers like me.) Or if you have no Idea what I just said in this short read, bring me your car and I will do it for you. I probably forgot some things too. So yeah a couple hundred bucks worth of material and tools, more or less, some time, and a car and desire and your in business. So find your shape, make your skeleton, and make it with fiberglass.