Here are some photos of the two shells that I have experimented with. These are corrugated boxes which have been stripped back to reveal the inner corrugations.  Then these have been filled in with plaster and the remaining cardboard has been burnt away.  The shells you see now are mainly plaster and some parts cardboard where it is needed still to hold together.  I am looking to see how I can remove the cardboard completely, leaving just a skeletal plaster shell.