like PitchBlackPZEV said, carpet, make sure it's unback carpet though, if you wanted something really nice you could get custom with fiberglass and all that, or if you dind't like that idea and wantd to paint the actual box you could sand it down really reqally good anduse some bondo to smooth it out anymore for a really smooth surface as if it were fiberglass, but then of course you don't have the custom fit of custom fiberglassing.
If you paint fiberboard (which shows through regardless) no matter how many coats you put on, it looks like s&^t. if you make your box out of something else (I'm sure it would be something revolutionary) it's fiberglass and you don't need paint. do something custom, with matching carpet, neons, whatever you want....
you could actualy make an mdf box look just like custom fiberglassed and painted work. it just takes patience, i'm talking sanding from 80 to 120, putting bondo on, sanding it with 120 to 240 to 300 some, another coat of bondo, sand down more and more until at about 600 or something and prime it, sand it to 600+, more primer, sand again. it's alot of work, probably not worth it though, but it's possible to make it look good.
I just really didn't want to do the whole carpet (not that it's hard). I was mainly looking for ideas on paint jobs. I'm not going to enter any comps w/ my car, but I wanted somethign different than carpet.