This is what I do. Take a Gallon milk jug or half gallon jug tie a line to the handle with a weight on the end of the line. Sort of like jug fishing. The weight keeps the horses from knocking it out of the tank. The wind moves the empty jug around and the floating jug keeps the tank from freezing solid on top.
could build a cedar wood box around the tank, fill with sawdust between box and tank. Then perhaps float a piece of foam on top the water with a hole for them to drink from. But when it gets really cold and stays there awhile the hole would freeze over and then you might be breaking the foam along with the ice, but that might take more cold than we get in TN. And the ice would be thinner and much slower to form with only a small hole for the cold air to work on chilling the water.
I have frost proof faucets next to the water tubs and just break the ice dump it out and refill.