The socks were going to be my suggestion.
Many people switch to crocks over the winter because sometimes the problem is not just the water in the bottle freezing. But the metal nozzles often freeze first. So a crock ensures that until the water is frozen, your rabbit can still get water. Plus, it may not freeze as fast in a crock...I'm not sure because my rabbits are in the garage over the winter and it rarely gets down to freezing.