Good bench substructure design enables the use of thin plywood. My layout has 180 square feet of inexpensive 3/8" thick plywood, and since it was built in 2008 it has not warped.
The substructure is 3' x 6' bench modules made of 1"x4" pine with 1"x4" supports across the 3' width 24" on center. The legs are 2"x3" fir and braced. The modules are clamped together, not bolted.
The plywood has a covering of 3/8" thick foam with a ground cover of sawdust put down in a liquid mixture of white glue and water. There is no doubt that the plywood has gotten wet while I was putting down the ground cover. Also, the 3' x 6' bench modules I use have supported my 200-pound weight without cracking.
I can lift every one of my 3'x6' lightweight bench modules without getting a hernia!
By the way, when I want some warping to occur to make the ground cover interesting, I put down some corrugated cardboard and wet it. It warps nicely as it dries, then I add ground cover.
