To true! Get a proper lid. I had one of mine go through egg crate so if they are small don't count on that stopping them, and larger one hurt themselves on in too so something smooth and clear. Glass, polycarb=(has poly right in its name), Plexi, lexan are all good options. Look for the polycarb they build green houses from it comes in different thicknesses and is pretty ridged. A 4x8 sheet of the thinner stuff at menards is less then $20, and like I said its ridged and transmits 90% of light...Which I thought wow that's cutting out 10% but standard glass cuts more then that. While not as good as open top for lighting it way safer to stop carpet surfing.(not a safe poly pastime)