FWIW, they are very good for tank dividers, but they are also handy as cut-to-fit covers for a tank as well
my tanks were regularly runnng about 5-degrees warmer than I wanted, but when I replaced my glass tops with egg-crate plastic tops, the problem went away almost immediately . . . yes, I have to top-off the tanks more often due to increased evaporation, but it's well worth it
also, the egg crate doesn't need to be wiped clean like the glass tops did, and it lets me feed the fist directly w/o having to open any lids . . .