I’ve considered keeping the makings for one in the car.
One large container candle will work. I use them to heat our boat at night. It really is amazing how much heat they put off.
When I was in Alaska, 45 years ago, we were told to keep at least TWO cans of STERNO and a STERNO stove in the car in winter.
Blankets, hard candy, stero, matches. Add a sleeping bag and a few plumber’s candles.