Originally Posted by Brookster123
The poly will warp, you can use eggcrate or get a sheet of 1/2" acrylic and it to fit. You could use thinner acrylic but you'll need clips to keep it from lifting.
OK, I've done some quick looking for acrylic sheets, the 1/2" is tough to find in the right size(looking at 12x24 so I can just trim it down and it's close to the right size for the tank; I'll need to cut off about a half inch in each dimension), and seems a little pricey.
Would 3/8" thick be workable to hold shape? It seems a fair bit easier to get hold of, and much nicer on the cost end as well.