yup, just stretch that shit out, i use fleece, staple it down to the mdf then just start loading the resin on with like 3-4 layers like widebody said. then instead of using body filler, i spray a white or purple gel coat with a spray gun because then there is no much sanding at all left to do and its alot stronger than any body filler. i was always afraid of some bondo or something cracking with some decent hitting subs so i use gel coats instead of filler after i apply all the glass. usually though, your first 1 or 2 boxes turn out like shit since yuor not really sure of what your doing, atleast thats what hapened to me, lol.
heres what one of my many of boxes i have made turned out like...






GOOOOOOOOD LUCK !