Ok all you rack builders/buyers out there! Time to give some of that experience away.

I have been mulling over plans for a 30 box corn hatchling rack and have come up with the following:
two types of box(store didn't have enough of one kind) Rubbermaid 'take along' rectangle at 6.5x9.5x2.5", and a generic 25oz square at 7x7x2.5". With lids on.
5 shelves high, 5-6" space between each shelf, 6 boxes to a shelf.
Shelf dimensions of @46"x11"

Here's the issue: What material should I make this out of?? I've seen great racks made from the plastic utility shelf units found at WalMart and Lowes where the legs are cut down, but those would have set dimensions. Should I use those or should I build it myself? If I build, should I use poly'ed plywood? I want it to be light, so no melamine.

Give me your suggestions, oh rack people!