Coyote, you can use Glad Press N' Seal, SaranWrap, or even cut up a black trashbag and tape it to the majority of the screen. You can cover the entire lid, leaving smallish triangles cut out at each corner, and a circle cut out that is a bit bigger than your lamp(s). Hopefully that can help save you some money and it won't be "as" hard keeping temps and humidity up in a glass tank.
To keep more heat in, you can cover three sides(except for front) with ConTac Paper covered cardboard. ConTac paper is the patterned pretty stuff that is stuck onto kitchen shelving. There are alot of different patterns, so hopefully you could find something you like.