I’ve had success wrapping the 5 gallon bucket in a big thick wool blanket, bungee cord around it and blanket over the top and put plastic over the top of the lid that screws on the bucket. I fill the bucket half full with super hot water, then put it in its place in their run, (on a towel on top of three stone pavers to lift it up off the ground), and bring a Watering can full of super hot water and fill the remaining 5 gallon bucket with the hot water and cover back over with the blanket and plastic to keep the blanket clean from the chickens sitting on it. Last winter that worked well and I’ve just got it set up for this winter. (I have the horizontal nipples that come out the side, near the bottom). The blanket seems to be enough to keep it insulated from freezing, but usually our temperatures only go to mid 20s here in the Pacific Northwest.
I’ve also filled a plastic water bottle with hot salted water and let it float around inside the 5 gallon bucket. This seems to work well.
Have any of you tried that and has it worked for you?