The rice bag is a good idea in theory, but since it isn't washable I would worry about urine on it, and chewing. When I have used them with my cavies I just put the bag in the pouch of my hoodie sweatshirt and put the pigs on top of it, if they seem cold. When I've tried putting it in the cage with them they freak out and don't go near it.
A better idea is a snuggle-safe disc. Designed specifically for small animals like cavies and hedgehogs, it will NOT get too hot and is 100% washable and chew proof.
Amazon.com: Pet Supply Imports SnuggleSafe Pet Heating Pad: Kitchen & Dining
ETA: I have seen them sold for as little as $22. So definitely shop around.
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