"
I don't have the specific reference, but casting-grade polyurethane rubber
for making gaskets and such is, or was, available in three different
durometers that correspond to a wide range of gasket types. I can be cast
in Plaster of Paris, so it's very easy to work with. It's been used for
decades to make reproduction parts for old cars.

Maybe someone else can help you with a link.

--
Ed Huntress


here are two hints

1. Lynn Steele rubber makes a wide range of repro rubber parts (not
particularly cheap) for old cars - I've used a lot of their stuff, generally
it is well done and fits

2. Skinned Knuckles Press publishes a book on casting small rubber parts