I'm using a product called "snuggle safe". It is a round pink plastic disk filled with some kind of gel, made for cats to lie on. You microwave it for several minutes to heat it up and it stays warm for hours. It fits perfectly under my water containers--which are white plastic with red pans on the bottom, and keeps the water from freezing for several hours, depending on how cold it is. I put one under the water in the coop at night, and the water is still unfrozen in the morning. It is just on the edge of freezing here usually, but tonight it is supposed to get down to 13 degrees, so it will be interesting to see if it still lasts through the night. Last year I used concrete pavers that I heated by the fireplace insert, but I think this holds the heat longer. I looked at heated water bases, but felt they were too expensive for what they were.