For rimless tanks (w/Eurobrace), I was instructed by the tank builder to us 3/4" plywood (actually I used Medium Density Overlay) and 1/2" Styrofoam (the sheets a home centers) to absorb imperfections and provides good "crush". I use that on my ~260G w/Metal Stand.
For rimmed tanks, I believe only plywood is advised, and, no styrofoam.