1/4 inch thick is a lot. For a hand lay-up, you could get the strength you need, and a lot less weight, with a thinner laminate. Generally, fiberglass is stiff but a little brittle, and other fabrics, like kevlar, are much stronger...ie more resistant to inpact. The reason comercially produced products like truck toppers are so thick, is that they are made of sprayed chopped fibers.
For your purpose, hand laid sections of glass and kevlar (or nylon cloth) would work fine. You would have to seam them together to get the box you wanted, and maybe gel coat the outside if you want it to look smooth and finished. A good source of materials would be Plasticrafts (it might have been renamed to Plasticare) but it is about a block west off of South Sante Fe somewhere in the vicinity of Oxford...in one of those commercial strip malls in Englewood. They would have everything you needed, including advice.
Smelly work, but easily enough doable.
Sascha Steinway TLCA # 20217
1977 FJ40 "Ripper" (because it shreds!) (What is a "shread"...Steve Crase)
1988 FJ62 "Dino" OME, my next project
2000 4 Runner, my daughter's truck, thanks Dan!
2013 F-150 "Black Ops"