In Montana it gets cold occasionally. To keep the water from icing up, I found a tin for twenty-five cents and a lamp for fifty cents at garage sales this summer.
In the tin, I drilled a 3/8" hole. I took the lamp assembly apart. Then I reassembled the lamp assembly inside the tin. The screw-on fastener on the bottom of the lamp assembly is on the outside of the tin, which keeps the 40-watt bulb stationary in the center of the tin.
I have both plastic and metal waterers. I have read that the tin works for both plastic and metal, so I don't know which I will do. Or, I may make a second water heater tin. Did I mention that I garage sale... I have enough parts to make one more heater.