What about memory foam on one of those heated pads that should make the hips better all warm. Soft so don't hurt hips and heat make it nice kind a extra.
the memory foam beds are pretty thick, i'm not sure the heat would go through from underneath. but they do make heated inserts for beds - that may work.
and by all means, i'm sure your "pup" would enjoy it. some are like memory foam chips, i'm not sure how that would be. BUT if you want a nice thick foam bed (not memory foam) and your dog isn't distructive, you can go to the craft/fabric/upholstry store and buy some really thick foam. I've done this for an insert in a bed