A Pit Shelter with coal beds, here is the generic form: Dig a grave, long enough for your body, plus a foot or so extra, and deep enough so you can lay down, but can bend your knees without touching the ceiling (I like lots of roll-over room).

Then dig down another 6-8 inches for the hot coal bed. Digging takes about 2 hours with a stick. Start a fire in the pit and burn hot for 1-3 hours, depending on soil moisture (burn time not included in total construction time).

A hot coal bed on the surface will produce steam when the dirt is put back on it, but in a pit shelter you can cover the coals with the now-dried dirt from the pit walls. The dirt puts out the coals, what you want is the hot mass of the ground. This covered, put a roof of sticks/logs across most of the top, except for a narrow doorway at one end. Cover the roof with debris to fill the cracks between the sticks, then cover with earth. Properly constructed you can sleep in shorts and a tee-shirt, without ever closing the door, down to about 20 degrees F. A coat draped over the hole is sufficient for a door if necessary.

For successive nights, fill with debris (an underground debris hut), or make the pit extra wide and bring in a row of football sized hot rocks along one side.

There are unlimited variations on the pit shelter, hybrids with other shelters, some partially above ground, door at the end instead of straight up. Utilizing natural pits can significantly reduce the dig time.

My favorite is to move into a pit created where a tree has fallen over, pulling a plug of earth out with it's roots. (This can only be done in damp weather, otherwise the fire may enter the roots of the tree and emerge days or weeks later.)