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Keep the packs warm at any cost, that is key. Some thin foam packing (sheet type) or even a pair of kids wool socks are a place to start. And as sikes said, they'll generate their own heat, but I do recommend preheating them (I have done it many times) to just over 100F. A heating pad or similar is fine. They will have serious punch, and as long as you keep moderate load on them the temp shouldn't drop much over 10-15 minutes. I would not let them get below about 80F, so check temps often.