Rabbit's water freezing in the winter?

I have been a proud rabbit owner since 1998. Started out with one and grew to six. Anyway while you reading this don't tell me that rabbits are the most abused rabbits or that them being out side in the winter is horrible, because being outside in the winter is actually better for them they like the cold because guess what they have fur. Now back to my question i bought insulaters for the water bottles the only problem is now the metal mouthpiece that they drink out of freezes. The bottles don't but i still have to thaw them out every 4-5 hours a day. I don't mind doing that, but if there is a way to prevent the mouth piece from freezing OTHER THEN MOVING THEM INSIDE i would like to know thanks to all the people who answer and rabbit breeders who help.