Start by digging a small pit in the floor of your shelter, a little bigger than the bowling ball sized rock that you will be using to transfer heat. Dig the hole to match the rock’s size and shape, and find a flat rock to cover the pit. Make sure that you get your two rocks from a dry location (water-logged rocks tend to explode when heated, so do not use rocks pulled from rivers, streams, and ponds). Ensure that everything fits together well before you heat up the stone, since a 1200 degree F rock isn’t a fun thing to juggle. You could even recess the hole of the pit surrounding the cover, so the flat rock sits flush with the dirt floor (not a trip hazard). When it’s time to use your set-up, heat up your pit stone in a fire for about an hour (but don’t heat the lid stone), carry the stone to the pit (a shovel works well), and drop it in. Seal the pit with your flat stone lid, and bask in the radiant heat that will last for several hours.