Similar to what TK Bunnies is saying, I went to my local Home improvement store, and bought a few of their Odd Ceramic Tiles. They are very inexpensive. I keep a couple in the fridge and put a couple in their cage and just switch them every so often. They stay quiet cool, and the buns love to lay on them. Just a suggestion.
I have also used Frozen water bottles, but I find they make a huge mess of the cage as they de-thaw, and the rabbit fur gets stuck all over it. My fav is the Ceramic Tiles.
Crystal