Cement canvas. Any shape bag made of hardening fabric can be inflate until
set.
This is old technology with a lazy twist that may garner interest. Tvo used
to write about it from time to time. It is similar to ferrocement. Create a
form, dip towels, burlap, or canvas in a sloppy cement slurry, then lay the
cloth over the forms. Hard tomorrow. Add insulation, do again. Add final
mortar/stucco layer with color if desired. Any shape is possible.