@Chicken girl 15 I was going to use a birdbath heater in a five gallon bucket with nipples, I hear they work great. If the nipples freeze up in extreme cold it they say holding your hands on them for a little bit thaws them out, the water in the bucket doesn't freeze. I hope to have a couple next yr. Rubber pan will do this yr. I have mine a couple inches into the litter, only froze over night, grounds not froze yet. The other coop I have it outside where the sun hits it. Both only had a thin layer of ice on the surface from 7am to 1pm when I left for work.
I'm not worried, last two winters were subzero for months, I can deal with a few cold days this winter, going to be between 35 and 50 for the whole rest of the winter this yr.