I got tired of multiple trips to break up ice last winter and decided to use an old cooler that was about 12 in tall. I had it buried in dirt in the chicken run with some straw at the bottom and around the edges. It stayed unfrozen most of the day when I filled it with very warm water. I used the bungee cord to keep the lid of the cooler propped open just far enough that chickens can reach their head in and get a drink of water.
Adding some wind blocks (plywood around the bottom of the run, and some bales of straw inside the run) added some additional protection from the cold and keeping the water from freezing as fast.
I like some of the ideas of compost surrounding a water tank and might switch to that later
