One of the most helpful tips I got was to keep your water bowl in the middle of the tank. I was having no problems with heat, but I couldn't keep my humidity above 40. Also, get a misting water bottle, and mist the tank every now and again. A 20 may be a little big, atm, but you'll want the space, later. Balls may like to be in small dark spaces, but as your friend gets bigger, they'll need bigger hides. I've got a 20 long, and am thinking of upgrading to a 40, bc my guy has gotten so big, his hides are almost too large for the tank.

As others before me have mentioned, a UTH is going to be a huge help with heat management. I've been told ceramic lamps tend to leach humidity, and should be avoided.