well first make sure you get the specs for the internal airspace for the particular sub you are using. then, measure the space you are putting it in to make sure it will fit. then cover the floor area that you are using with saranwrap, or something else that will contour to the cars carpeting, but will be able to be removed. then if you can get ahold of the proper equipment (i own the stuff, but i fix fiberglass boats) spray some fiberglass over the saran wrap (or whaterver you use) in the exact space where you are putting the box, and make sure to make it thick. if you're like most people, you wont be able to get the equipment to do that, but it can be done manually. go to your local hardware store and get some fiberglass matting and a can of fiberglass resin and hardener. then make the layers by hand, its a bit more tedious, but still affective. then build the top and sides of the box with mid or high density particle board (for most subs smaller than 15's mid density seems to sound the best, for 15's or 18's use the high density). try to make your measurements as close as possible, but if the sides dont match up perfectly squarely dont worry, cuz you'll take care of that later.

Now if you bought a cheaper type sub, then it might come with a template for the width of the hole for the sub, if not then i recommend trying to find one, but if you cant, take a tape measure and measue straight across the sub from one mountng hole to the next. then cut a string exactly half the lenth of your measurement and screw one end of it approximately where you want the center of the hole to be. then hold a pencil or chalk at the other end of the string and rotate it around the screw to get the perfect size hole. then drill a hole somewhere within the circle and use a jigsaw or whatever to cut out the hole.

then remove the dried fiberglass from the floor of the car, it should remove easily because of the saran wrap under it, and the idea of the fiberglass is that the bottom of the box will then be moulded perfectly to the surface it will be sitting on, cuz its been my experience that hardly any trunks, seats, or the area where a back seat used to be are parfectly flat and level. then fit the top of the box onto the fiberglass, then screw it to the box. then you can trim off the excess fiberglass with a saw, or in the case of most of my boxes a subatomic warhead. that fiberglass gets f*&#ng hard. since most peoples amateur or home made boxes arent perfect at the edges they arent air tight. this is easily fixed by using bondo or any other body filler. simply bondo every seam, but take care to make sure that its smooth. you may even have to sand it or file it down a bit, but the bondo will make it air tight, it wont crack and vibrate off, and it doesnt hurt the boxes accoustics a single bit. after that you will have a perfectly sealed box that fits the floor perfectly, oh and id recommend carpeting it, thats the only difficult part of it all, so i usually just cover the whole thing in vinyl, or cover the entire surface with bondo then sand it really smoothe and paint it.

i personally make all the ebclosures for my car purely out of fiberglass, bondo, and vinyl which sounds awsome and looks superb, but i have a lil better resources than most. but following my steps for making a sub enclosure should leave you satisfied, and should still cost less than buying a box. mid or high density particle board should run from 20-30 dollars, bondo is about 6 (if you buy a small can, fiberglass and resin should run between 10 and 15. hope this helps, if not, well it always worked for me so hopefully it does for you too



oh, and for carpeting, if you find that too much of a pain, you can also buy some fleece from any fabric store, soak it in the fiberglass resin and just stick it to the box. it looks kinda cool and its pretty easy to do


oh, and if custom fit fiberglass for corners or whatever is what yer looking for, thats too long of a procesure to type, so you can pm me if youd like.