My pets LOVE the heat reflective pads! I always use the size sold for cats. From experience, I will add not to dry them in the dryer, even though they say you can. It makes them very loud and crinkly! Also, check often to make sure your pup isn't chewing on them, as they have a metallic material in them (like an emergency blanket) that probably isn't very good to eat!
Oh, and there are discs you can put in the microwave that stay warm for a long time. A stocking full of dry rice works too!
First time Chihuahua owner with many years of dog experience!