Humidity can be an evil thing. I have used giant bowls of sphagnum moss, humidifiers, spraying, all of course with keeping the top of my tanks 75 % sealed. I understand how much it can suck but you do need a little air circulation so just uncover the spot behind the lamp (about 3 inches by 3 inches looks about the space you have there).

I use a clean damp cloth changed daily on the top of the tank and I find that works best. Mind you the humidity in my apartment is about 25% (cough, cough) and my tank is 3 feet long so it depends on each individual set up.