>>657532>>657582The mylar will work, or just keep some cordage, tarp and blanket in the car.
Those will have other uses and be handy at times when S doesn't HTF, such as you get caught in snow, car breaks down, any other reason you would be better staying with the car.
You probably already have them so don't need to go buy anything.
The GHB I have in my car is in a 3 way deployment bag, canteen holder and map-case. That will see me right if I can walk home that day.
Woollen picnic blanket and tarp in the car will see me right till I get to where BoB is.
