For my money? Balsa wood and brass tacks.
About a week ago I picked up a 6"x20" sheet of thick-ish balsa from my local hobby shop (mind you this was more of a model trains kind of place than a warhammer kind of place, almost no game stores carry balsa in my experience). That was enough to cut out three square planks to put my Saurus blocks on, and the extra was enough to make lips around the edges so that they don't fall off. Then I secured the lips to the trays with some little brass tacks (mind that they're sharp if you do this yourself, dull tacks will make balsa fibers fold up like paper instead of cutting through). Total cost? About eight USD. Plus, being wood, they're dead easy to paint. Haven't gotten to it yet, but I'm thinking of color-coding them by unit.