Easy fix - Switch to the serilite or rubbermaid setup. It doesn't look as nice though. This is a true function over fashion setup.

Other fix - duct tape, extra thick platic wrap. Cover the top of the whole enclosure, except for a place to set the heat lamp on the screen. With this addition, my setup would be identical to yours, and my humidity floats right around 55-60%. During shed I mist a few times daily and it raises it to about 80%. I feel like this is best of both worlds; just takes a little bit more work.