I had a similar problem but was informed of an inexpensive trick to use. Take aluminum foil and cover 3/4 of your screen cover. You can make cutouts for the heat lamps if needed. I have a 20 long tank that I keep my first Ball in and was never able to keep the humidity above 40-50 %. Since I put the aluminum foil on the screen, it stays around 60-65% with just a large water bowl. Also, I don't keep the red heat lamp on as much as it seemed to hamper my attempts at raising the humidity. The UTH keeps the temp around 78-85 degrees alone.

Forgot to mention that I used electrical tape to fasten the aluminum foil to the cover and keep it tight.