A little more expensive (than foil) route would be to either get some plexi cut to the size of your screen and drill some vent holes through it so that there is still some air that can get in/out, but keep the majority of humidity in. I didn't see where you mentioned what type of substrate you had in there that molded. As far as I know aspen is one of the only ones that will mold. You could trpress mulch or orchid bark...
On just a random side note it may be beneficial for you to cover the back and two sides of your tank to make your guy feel a little more secure.
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