mpkeelee - i think i discovered what HAS to be the cheapest, easiest way to provide back up heat...ready for it?
here it is...

a hot water container wrapped in a pillow case or a t-shirt.
(note: a shallow square or rectangular shaped container will be better then the bowl-style ones.)
put this on the warm side with the warm side hide on top of it.

the hotter the water you put in it, the longer it stays hot. the pillow case or t-shirt shields the snake from any potential burns.
when it cools down, simply replace the water.

i came up with that when our power went out after a particularly severe thunderstorm and was struggling to figure out how to provide back up heat. i was almost resigning to the idea of having to spend alot of money on the instant, short lived hand warmer packs but then a light went off in my head and i ran to our tupperware drawer. now i have a specially designated container kept next to my girl's terrarium just for these such occasions.