For my snakes I have those pocket warmer things; they heat up instantly, sort of. I have 2 hour ones that heat up quickly and then I have 48 hour ones and also some 60 hour ones. Put them into sweat socks so the snakes don't burn themselves on the heat pack.

As for incubators, I've read on the forum that people use bottles of water in the bottom of their incubator that will help to hold the heat for awhile. Back up power helps (like a genny).