Here is the first casting, and hooray, there appears to be no air bubbles inside.
The curved surface isn’t great, but was no more than expected considering it was moulded from a clay plug. It will need a bit work using very fine wet and dry sandpaper followed by polishing with a cutting compound to bring it up to the glass clear surface I need. If I’d been able to find an item as the plug that had a polished surface then that is how the cast would have come out because the silicone rubber mould matches the plug’s surface exactly.
However, I’m really pleased with the way it’s come out and very excited now.