I live in Sarnac Lake Ny. Its the true upstate NY. The town itself is usually the coldest place in the US a few times a year. We had a week straight a few years back of -40. We keep a generator but not for heat, just for the electrical stuff for my dads tanks mostly.

Hand warmers can be great, they do get real hot but only last for about 5-7 hours. Do you have a woodstove? I know I can get my woodstove cranking when I want, and that kind of heat is hard to beat. Plus you don't have to pay for oil.

For a temporary fix I'd say the handwarmers are nice, but limited. They are more for keeping in gloves and boots so if they are in a cardboard box they may not generate a huge amount of heat for your herps.