i suggest you build you own boxes... just get a program like winISD, and figure out what size box you need, then plan it out and build it yourself...
once you figure out what size box, play with the dimensions till it comes out right...
(L"xW"xH") / 1728 = box size in cubic feet
then just remember to use the inner dimensions of the box, not the outer, if you use 3/4" mdf, just measure the outter dimensions, and subtract 1.5 from each one. (L, W, and H)