Find a nice lid off a tin or bottle that you think would compliment the display. In this case I used the lid off an aluminum Illy espresso tin.
Take a sheet of thin styrene, or failing that a piece of thin cardboard will suffice (be warned that if you use cardboard, the surface of the plaster will have slight ripples from the damp cardboard, whereas styrene will cast perfectly smooth). Wrap it around the lid tightly to form a cylinder, secured with masking tape. It doesn't matter which way the lid is oriented - judge for yourself which way looks best for the display. In this case I turned the lid upright, where there is a smooth surface with a slight ridge around the edge. This allows the plaster to be removable, but still easy to seat because of the ridge. Cover all the seams with masking tape to prevent leaks (don't worry about a perfect seal around the lid, a little overflow is fine and can be fixed later).
Mix up a thin mixture of plaster of paris. The consistency should be quite watery, so it will flow smoothly. Immediately pour the mixture into the cylinder before it thickens and stir it a bit to bring the bubbles to the surface. Let this dry overnight.
The next day remove the 'mould' and check over the plaster. You'll have to do some carving with a sharp knife - there will be a ridge along the top edge where the plaster stuck to the walls of the mould, and if there was any seepage around the lid at the bottom you'll need to trim it off. Prepare the surface any way you want, either carve it with scribes and knives or mix up some fresh plaster / whatever and build up groundwork on top of the cylinder. If you used a flat lid with the plaster on top, the plaster should be removable, making it easy to prepare and paint separately. If you filled the lid upside down, the threaded surface inside the lid will hold the plaster block in place.
You should end up with something like this:
Cheers
JC
