I have aluminum foil on the top of mine. I have about a half inch gap at the end of the hot side, about an inch on the cold side end, and about 1 1/2-2 inches in the middle. The ones on the side have to be there because of the clips, and the one in the middle is the biggest. When I need to up the humidity I put a plastic bag from the grocery store (not the kind they give you when you buy stuff, but like the kind you would put fruit in in the fruit section) over the top and a towel on top of that. I always leave the gap on the cool side open no matter what.

The aluminum foil has worked very well so far. I want to upgrade to plexi eventually. It works well, but I think I have too much air gaps in the top. It doesnt hold my ambient air temps high enough. My parents A/C screws this up also, itll be easier to maintain at school. My ambient air temps are like 76-77*.