Heat rocks are bad and can cause burns. Glass tanks are not very good to keep balls in, as they require a lot of modification to keep everything right. I'd recommend getting a professional PVC type cage (animalplastics.com, boaphileplastics.com).

In the meantime, get some rigid insulation foam and cover three sides of the tank. You may also want to get a red bulb as suggested above. You can cover most of the screen top with plastic wrap and a towel to help hold in heat and humidity.

You should definitely get a thermostat (a Hydrofarm one on eBay sells for about $30) to regulate the heat pad, as unregulated they can cause burns. You can also find an Accurite indoor/outdoor thermometer that measures humidity as well at Walmart for about $12. The thermometers that you place on the sides of the tank are useless, since they are not measuring the hot spot, rather, a few inches above it, which can be several degrees different.