I want to try an overnight minivan camp this winter for sunrise photography, I'm trying to come up with a cheap way to keep myself warm overnight in -5f or so. I figure I'll do reflectix on the windows for insulation and sleep in my 0 degree bag on an air mattress with several other blankets, but I want an actual heating element to either heat me directly, or to heat the space. The popular direct heating method is an electric blanket, which requires a battery bank that costs a few hundred dollars, so that's out. For warming the space the common method is to use a propane buddy heater, but that releases moisture and apparently makes things uncomfortable, so that's not ideal either. One idea I had was to boil a bunch of water before bed with my portable butane/propane stove and fill up a few hot water bottles, then store them in a dedicated plastic cooler for insulation. I'd take out a couple at a time and hopefully be able to go the night just grabbing new ones when the old ones get cold. I figure this would be less annoying to do at night than changing out a propane canister, for example. Does this sound like a plausible solution? Would the cooler be enough to keep the water hot all night?