Casting in plaster
Melt some wax, and with a big brush coat it over the relief of which you want to have the cave, as if you wanted to paint it. And a light crust will be formed upon which you will cast plaster, to give it shape and consistency. Afterwards, in the cave of the wax, cast your plaster as well to get the relief, and it will come out very easily, because of the wax. This is done more often for large pieces in haut relief than for others.