I found a tutorial on easy plaster casting.
The point is mainly to create the mold of a star or the like from aluminium foil and to cast it with plaster.
Even if it is filed under ‘kids crafts’, I find the idea interesting.
I did it once, but I can’t find back my item ;-(
At that time I made a mold with a rounded bottom.
To fix it thoroughly (to prevent the plaster to ‘run away’) I put the aluminium foil mold into a layer of flour.
Sand would be as well.
Special Shape Castings