Thanks for all the ideas! None of my cats are wire biters, so I tried the human heating pad thing. Bo loved it! I'm going to look into the heated disc thing, the problem is my cats don't have their own "beds", per se, they sleep wherever they want.

Like this:

THis is funny because Rocky is usually so timid around the cats. Hanna was there first and left him room - he climbed on with her! We got some new memory foam beds for the dogs and piled the older beds up, and it is now the favorite bed in the house.