Yes, I agree that using flexwatt inside an enclosure is risky business. Due to the shock factor, and due to the fact it will probably get wet at some point.

Use a zoo med reptile pad and punch a hole in the back wall to run your cord out. Or as pointed out use a ceramic or other heat emitter. Just make sure you have a guard in place to protect the snake from burning itself on the heat emitter and make sure you use some type of thermostat.

Best of Luck!