Covering about 3/4 of the screen with something like aluminum foil, aluminum tape, plastic wrap, or a piece of plexiglass will help keep the humidity in. Another thing that works is to soak a towel with water, wring it out so that it is wet but not dripping, and lay it across the top. You will probably need to change it out about every 8-12 hours. If it gets the humidity too high, just fold it back a little until the humidity stabilizes where you want it...