I've finally got the mould sorted.
This has involved getting an existing cover to take a mould from. The refactory fibre paper used to make the mould was soaked in hardener and formed around the master. I left it to dry for a week while I went on holiday, but it was still damp when I came back. I had to remove all the weight packing the fibre to the master and let it air dry for days. When it could support itself, I put it into the kiln to fully dry and cure.
It came out of the kiln firm, but it needed sanding to final shaping and to smooth the surface.
This is the mould sanded, hardened a second time and now washed with a separator to keep the glass from sticking to the mould. The advantage to this kind of mould is that it is light weight and reasonably easy to shape and adapt to a number of simple shapes. Its big disadvantage is that it is pretty fragile. Also it is not cheap.
The tests have been made for the diameter of the discs and the temperature required for an appropriate slump.
These are the two test pieces together with the original piece. Now I'm ready to produce two of these a day for the next week.