The first time I tested this mold, I strapped around the sleeve with bike inner tubes to hold the exposed Vinamold half in. This did not work effectively and the Vinamold bulged out. When it bulged, I pushed it back it by hand, which cause the plaster to spill back out the top pouring shoots and cover the existing plaster sleeve. I had to open the mold and wash it away while it was still wet, otherwise it could permanently distort the sleeve.
The second time I tested it, I strapped a bit of wood (the piece I originally cast the rubber off of) to the open half of the Vinamold to create more pressure / tension.
This worked much better, and I had no spillage or leaking of plaster from the mold.
Unfortunately, I mixed the plaster too thin, causing it to leak down the middle of the mold, and causing a ‘crystal’ effect. The plaster marbles were not as solid as I would like them, and had a pearlescent effect. Luckily this is very easily fixable!
It appears that there’s no obvious problem with the alignment of the halves, and that this mold is the most successful so far. I am very pleased. Hopefully the next time I use it, I will make the plaster the correct consistency and the results will be even better. They are still not perfect, but at the moment I am unsure how I could make a more successful mold with my current skill set.