The reptile shipping heat packs are very good. I always have some on hand. They don't get hot enough to burn, and they run for quite a long time. I've used them on camping trips and they've stayed warm for 3 days just as the packaging indicates. They're worth having - just double check the expiration dates and use them up and replace them as needed.
Another good idea, but more expensive, is to have a battery backup for the lights (or just one heat light). These are typically designed for computers or servers but will keep a low wattage heat emitter going for a little while. Due to their limited run time, they're best suited for short outages of up to a few hours.