Heat baggies for after surgery

I'm looking for ways to keep animals warm post-op in a high capacity spay and neuter clinic for a shelter. Currently, the floor for the "beach" is heated, and volunteers sew bags of rice that we microwave to keep with animals until they're actively moving. We unfortunately don't have a budget for bair warmers or much of anything else really, and keep burning out the microwaves. We use a cooler to hold the bags after we preheat before surgery starts, but they always cool off too much and we only have so many.

Does anyone use anything different that they like? I'm looking for solutions that are replacing the storage to keep the bags warm, or replace the bags of rice: cheap, semi disposable, able to be moved with the animal from the beach to the kennel. I'm fully aware there may not be a better answer, but it doesn't hurt to ask. Any other fillings that you like using rather than rice (which breaks down after a while)?

Thank you!