In the hole you dug, so on top of whatever soil or sand you dug into and filled with stones/rocks. You fill the hole with water after the fire has died down and

In the hole you dug, so on top of whatever soil or sand you dug into and filled with stones/rocks. You fill the hole with water after the fire has died down and use the residual heat in the stones. You can't make a soup this way, but you should be able to make some steam -- I think.