Hi there,
I have never used the mylar, but I have made several quilts/pads/beds for dogs over the years. What I have liked to use, especially if your dogs a bigger, is a top layer of the "egg carton" foam for mattresses. It seems to be more comfortable for the dogs as they age and develop arthritis. Another insulating idea might be the heat resistant batting such as Insul-Bright. I have no idea which would work best, but probably the mylar. Or you could go belts-and-suspenders and use both!