My pasture is quite far from the barn and I do not yet have electricity down there. I conducted an experiment yesterday by filling a rectangular coleman cooler as well as a muck tub. Both were filled at 8:30am and temps were in the single digits. The high only got up to 12 degrees, but by the time I brought the horses in at 5:30, the cooler water was still completely unfrozen while the muck tub had a thick layer of ice.
I put the lid on the cooler overnight to see if it would remain unfrozen even with lows in the single digits. When I turned out this morning, the cooler water just had the thinnest glaze of ice.
Just an idea to try for anyone struggling with water in these extreme temps