what about making a "masonry"-bell from rammed earth for a RMH?
you could make a mold from boards and fill the void with clay-sand-mixture. and tamper it after every 4-6 inches. then add a metal-sheet on top and cob that in. you could leave the inner boards inside the bell as sacrificial mold.
i would expect that the rammed earth wall would not crack or shrink much, because in rammed earth you would use just slightly damp clay-sand.
how thick would that wall need to be?
my other thought was making a masonry-bell with clay-bricks. but that would take time for them to dry out (at least a bit).
i think, i should/could make a small freestanding test-wall. but i d like to hear your guesses on thickness.