I have found that keeping approximately half of the screen top covered in a double layer of aluminum foil and then placing a saturated hand towel in the middle of the top helps ALOT in maintaining humidity in your tank. I have had great success with this method so far and my humidity stays steady at 55-65%. I simply saturate the towel once in the morning and once at night when I turn their night lights on and all is good. Of course you will still need the bowl of water in the aquarium as well.
I will be upgrading in the next couple of weeks to fully enclosed habitats to further help in the minute controls of temps and humidity in my tanks.