I actually keep 1 to 2 shipping heat packs on hand for each of my snakes in the event of needing to evacuate or there's a power outage etc. 40 hour heat packs, in a paper bag, work perfectly for heating up an area of a tub for a while. Plus since they last about 40 hours, you can plan how many you'll need, and just pick up that many. I've never had to go more than the 40 hours with them, so a couple weeks of relying on them may get a little old...but I've found it extremely effective. I use them when we are out at presentations for the day as well when the weather is cooler.

-Jen