I use 1/2" plywood and those foam floor tiles for kids...they are about 3/8" thick and are usually 24" x 24" and interlock into each other!!

i had a 3x2x2 tank on a metal frame. I ended up going with a double layer of 3/4" then 1/2" foam. probably a bit over kill but I wanted to lift the tank a little higher.

Steve