make a box that fits inside the wheel well out of 3/4 mdf, then make a fake floor out of 1/2(or 1/4 mdf id available) thats the size of the wheel well opening, and place the sub accordingly.
fiberglassing would be good also, but rather overkill for a simple setup. if you really wanted to though, take the spare out, cover the whole wheel well as well as a few feet around it for overlapping, with a couple layers of alum foil and masking tape, and then lay down a couple layers of torn chopmatte, around 2-4sq inches each(found at home depot, the thick stuff, they come in large sheets you tear off as you go) over that. when that hardens, you get a perfect mold out of the wheel well, that you can then cut out a wooden top to fit with a opening for the sub, fasten that(screw in from the side) to the mold, as well as put a bit of fiberglass matte between the gaps in the wood and mold to airtight the summabltch, then a layer or two of glasse over the wood. sand that all down, add a bit of body filler, sand, and then cut out a new piece of carpet and mount the sub and amp and whatnot.
i think alot of people lack imagination when it comes to enclosures and what they think they cant do.
http://www.cardomain.com/ride/2090686/4 for my little customness.
