Home bargains used to sell a heat reflecting mat, it was small but for a fiver rather good. When Bob got old and arthritic I bought him a heat pad from eBay. It plugs in,I think it cost about £15 but he loved it. It had a protective cover around the cable and was listed as a puppy heat pad I think.
I'm on my phone at present but I'll have a look tomorrow and see if I can find it on eBay.