If you are going to lay things on top of the screen lid, I'd add a second screen cover - the kind they sell for regular aquariums. The screen lids on Critter Cages don't seem all that sturdy and laying things on top will undoubtedly shorten its life span. (And I haven't seen any direct replacements for sale anywhere.) Adding a second screen lid will give you about a 1/8 to 1/4 inche between the top screen and the slide-in critter cage lid. That way, the top screen lid will take the wear and tear, not the critter cage lid. It's not all that inconvenient, and they are cheap.