I'm currently using a hand towel which I wet in the sink and wring out once in the morning, once before bed. Mold shouldn't be a problem for awhile if you use this method, though eventually it will need washing. Are you using a glass tank? That's what I put my new BP in and I am seriously considering moving him to a tub. My CaliKing doesn't have the humidity issues like my BP does. I have ordered a heavier duty UTH for my boy as suggested by people on the forum since balls don't technically need a daytime/nighttime light (they live in termite mounds naturally) and belly heat helps them digest. I've heard that if you can keep the tank up to the proper temps with just an UTH, humidity is much less of a concern (those lamps dry out the air, fast.) You might give that a try. I used foam board to insulate the cage on three sides, as well, and that's helped keep my cage temp up with a lower level watt red bulb.
Oh, and as for the lamp, I only cover the 3/4 of the cage the lamp doesn't sit on. This leaves room for air to circulate as well.