Suppose all you are going to have is temprary shelter in the wild as you self-rescue or travel out of a disaster area. Maybe an improvised or small tent from your BOB supplies; maybe enhaced by getting up under a large bushy tree or in sheltering rocks.

You have to move and you need to get warm to recoup energy for moving, sleep, etcetera. Fuel for wood fire is available, but not much. Rocks may not be readily available. You have one water carrier container. You do not have much time each day to deploy your heating strategy.

What is the best and least elaborate way get adequate heat into your sleeping area for a whole night? What is the best and least elaborate combination of strategies to accomplish the goal?