I'm trying to come up with an easy way to sleep cozy in a sleeping bag on a cold night. My current idea is to boil some water before bed, fill up a few hot water bottles, and keep them in a hot cooler, rotating them out every couple/few hours as they get cold. Does this sound feasible to you? My bag is rated down to 15f, my scenario is van camping maybe down to 0f to do some astrophotography in the mountains.