The idea is to adapt the firepit to the size of a cooking pot which will always be the same. The main material is rammed earth. You will need two thirds of clayish mud and a third of minced bulck (in volume). All sorts of plants can be used : dried herbs, hay, straw, dried fern or twigs of bot broom.
Mix and knead the materials adding some water until mixture is a compact mud.
On a flat ground prepare the bottom of the firepit by packing a circle of rammed earth 2 - 2.5" high and 8" larger than the diameter of the cooking pot.
How to build the economical firepit
Sink three or four stones that look alike into this circle so as to make a stable foundation for the cooking pot and to leave some room for the fire. There should be about 8" in between the rammed-earth circle and the bottom of your pot.
You will then make an arched doorway between two of these stones, 6" large and 8" high in order to refuel the fire. You can materialise it with a stripe of strong cardboard or a bended metal sheet. Build a circular wall in packed rammed earth around it. The wall should be 6 - 8 " large. Make sure you smoothen both the inside and the outside of your wall.
The height of the wall shoud be around two fingers below the handles of your cooking pot, that way you will be able to manipulate it easily. Leave a 1" gap on the side of the pot. If you make two additional holes 2 - 4 " wide opposite the one you already made you will enable your fire to draw better and keep the smoke from escaping on the sides of the pot.
Using the economical firepit
Let to dry for 48h.
Fuel your fire moderately with twigs and small firewood. Put the lid on top of your pot. You will be surprised by the efficiency of this hearth.
If you use this fire alongside a good firetable you will always have a fair amount of hot water ready to use.