A lot depends on how many rabbits you're trying to keep watered as well. If you just have a few, having two bottles per cage is one way. Bring the frozen bottles in to thaw, take warm bottles out - repeat as necessary.
Otherwise, sturdy crocks. I buy cheap plastic dishes at the dollar store and thrift store. If they have a bit of flex and are wider at the top than the bottom, it's easy enough to pop the ice out. And if they break, oh well!
I just don't like the idea of electric cords near sharp little teeth. :|