If you want to do a lot of rats at once, get a styrofoam beer cooler, put the rats in it, and pour in warm water. You may have to do a water change after about 15 minutes because a lot of rats = lots to heat. Rinse out the beer cooler when you're done, and I wouldn't advise using it for beer afterward.
That's how I heat rabbits for the retics, but it can take a few hours to bring them fully to temperature even after being thawed in the refrigerator for a few days. Bunny fur is a great insulator.
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