While I was waiting for a delivery of plaster of Paris, I researched homemade alternatives. A number of sources recommended a simple mixture of flour and water for a similar result.
I mixed up a paste, adding some Brusho for colour. To create interesting shape and texture I poured the paste into an ice cube bag. Unfortunately, I clearly made the paste too runny as the mixture simply refused to set, despite leaving it for several days.
Nonetheless, I think there is scope to repeat this exercise, either with real plaster of Paris or with flour paste at the correct ratio. The paste filled the individual pouches well and using pegs, clamps or ties would I think have enabled me to get some interesting shapes. Watch this space for v2!
(Footnote: Flour paste smells terrible if it doesn’t set).