You might want to look at http://www.devcon.com/ they make a number of
products which may be suitable. I expect Dw Corning do also.

wrote:

Greetings all,

I would like to make my own rubber stoppers I have created a mold using plaster of paris,
I have tried epoxy and fiberglass resin but these end up being to hard does anyone know
what I can use to make softer rubber stoppers.

TIA SAL