I doubt that lying on top of a space blanket (aka emergency blanket) would do much to aid an elderly dog in heat retention. They are intended to be wrapped around the body to keep heat in, not laid on top of. Even when they're used for feral cat shelters, they're attached to the outside of the box, not inside. They are very thin, noisy, and not cozy at all. I also doubt that they are washable. I can't imagine any dog wanting to snuggle up on top of one even if it were covered with something soft.
I think perhaps a better idea would be the egg carton foam covered with faux shearling. There are many sizes pre-made and sold as pads for dog crates. My critters, dogs and cats, have always loved them and the covers are easily zipped off for machine washing and drying. My vet was the first to recommend them to me for a senior Sheltie many years ago. Entirely your decision of course.