If you do use supplemental heat, please keep an eye on it. A heat source in a small, closed container can produce way more heat than you might imagine. If you have one of those digital thermometers with the probe at the end of a long cord, you could stick the probe in the box and have the read-out sitting where you can check it from time to time.

But so long as he hasn't just eaten, he should be fine for a day without any extra heat. A bit of insulation around him, and tuck his box someplace secure away from an AC vent and he should be fine.