I loved this idea but I can't stand the thought of using anything with an AUTO OFF feature - I would never sleep! I rigged up a small experiment with an aluminum foil roasting pan and two reptile heating pads. Because I plan to brood my chicks outside in the empty coop it just didn't' generate enough heat to keep ME happy ( maybe the chicks would have been fine, who knows).
For a few dollars more than a household heating pad you could buy an extra large reptile heater. It's either on or off, no electronics to go wrong. One size comes with adhesive on it and they tend to be at least a little moisture resistant. Might be a viable option?
I don't know if you read comprehensively what people are saying when they refer to an "auto off" feature. This means the heating pad has a feature where it goes off automatically after two hours of operation....and these HPs we are using also have a feature that overrides the "auto off" feature so that the heating pad stays on continually....all the time...for months, if need be.
Unless someone lives in an area where the power goes off frequently, these HPs are not much of a risk at all. I've used them to incubate eggs for three straight incubations in a row without any issues...then used them to brood the chicks after they hatched.