I have used microwave heating pads and they usually only hold heat for up to an hour. I don't think it would do well for all day.
Lectro-Kennel Outdoor Heated Pads | CozyWinters.com
This one is a bit more expensive but it is supposed to be waterproof and chewproof. I would be really careful putting something that was not chew proof in with a puppy or any dog that chews while you are away.
There are several other heating pads for dogs on this site but this is the only one for a dog that may chew.