I have had, in the past, a moulding-rubber which melted at low temperature. Intended to make moulds for epoxy-resin casting,, IIRC it came from "Strand Glass Co." a stockist of Fibreglass mats,resins and rovings..
Unfortunately it was bright yellow. Maybe you could make a mockup eyepiece from Balsawood (light, quick and easy to carve and finish)
from this "plug" make a mould in Plaster of Paris....finally cast your "rubber" in the mould (which can be made in stages, divided by polythene or clingfilm, to allow dismantling fo removal of the plug and the final casting. paint the finished article with rubberised paint, maybe.
Alternatively,there may be some sort of liquid plastic which sets "soft"
There is also a "lost wax" process for casting. although we refer to Mouldings, they are essentially castings.
ISTR a you-tube video of a French Postman turned engineer , who built a superb scale-model of a Ferrari, around a metre long, it had fully-working engine,instruments etc. He
made exact scale moulds of the full-size tyres and I think he moulded the replicas himself.
last I heard, he had worldwide commissions for his models....he's no longer a postie!
