If anyone else has done this, they’ve been keeping quiet about it, so I thought it would be useful to share it here.
I made the top & bottom of the mould separately so all the parts could be printed without supports. I used up some filament I don’t like the look of — it’s supposed to look like marble, just looks grey to me. The green part that fits inside the cup is printed with TPU flexible filament to make it easier to remove. The mould design isn’t complete but it works well enough and I don’t suppose I will develop it further. The mould parts don’t have any locating pins and I assemble the parts by sight. Where two faces meet, I make gaskets from 2mm craft foam. The main mould & the flexible cup are held together with rubber bands stretched between hooked parts to take the strain.
I cast candlesticks in plaster of Paris which turns out to be a poor choice. The finished products are very fragile as the plaster is far more brittle than I expected. I tried various plasters and none were as strong as I hoped. If I was making these for sale I would probably cast them in resin which would probably require a redesigned mould.
I have experimented with colouring the plaster by adding 5% mica to dry weight of plaster. I am told mica is extracted by exploited workers & child labour, so it’s worth spending a little more to get the lab-made synthetic equivalent.