Hm...I know that the main reason for the thermostat is concern for the cage overheating, but perhaps a low wattage bulb would work. I'm really not sure about this, so I would wait for someone with a little more experience to come along and give more advice. I know LizardGirl wrote a really fantastic guide for how to heat cages, so maybe you could try posting on that thread or getting in contact with her. Sorry I can't be of more help.
