How to keep water in your rabbit's cage from freezing over in less than an hour? (ten points for best answer!)?

Not sure what 20f is these days,all of Europe uses centigrade.However from your description,it is very cold indeed.Warm water (not hot) ? .Alternatively,a space in a garage etc.,for the cage until things warm up.Alternatively,speak to the petshop you buy food from.Good Luck !