This was part of my nursery plant-pot mold-making project. I held previous shell-molds down with a brick on top. But that was not working well, putting the mold out of shape, making it difficult to pour the slip inside and agitate the mold after pouring to surface the bubbles. I found that the 3D printed mold can be held down by simply using a sticky clay slip (Plainsman Polar Ice)! I printed this one with a flange at the bottom for this purpose. I am pouring the mold on an arborite bat on the wheelhead of a potters wheel. This enables turning the wheel slowly and pouring the plaster into the perimeter slowly through a funnel, that produces a much better surface. If you don't have a super sticky clay you can make one by adding 4% Veegum to a Grolleg porcelain. Actually, Grolleg and New Zealand kaolins are both very sticky, so they could also work.
Nursery Plant Pot