[GT40Racer]

"Absolutely right - to get perfect looking on both sides would require a two part mould. At the moment beyond my skills....but who knows in the future."

To get perfect yes a two part mould would be needed. What I did for vintage Grand Prix tyres was to place the tyre on an 'O' ring and then pour the silicone for the mould. This created a lip, pour or syringe the urethane into the mould and then when take the tyre out you have a double sided side wall with a urethane 'O' ring which trims off nicely by spinning the tyre and a sharp blade