I'm currently using a glass tank with a screened over top and I'm not having any problems with humidity and temps. I guess the trick is covering as much of it as possible to keep in the humidity. The lid I built for my tank has an opening on two sides, on the one side I covered the screen completely with tin foil to keep in the humidity. I'm not sure about the location of your tank, but with mine the top is difficult to see and the tin foil is barely visible so it looks nice.

I'll try to snap some pictures and post them for you to see. The lid construction took a couple of hours with a friend of mine helping and it works great. Hopefully tonight I'll be able to get some shots.