What you want to use is two-component silicon rubber.
The medical type is shiny and sold for example here.
The artistic type is used for molding, not shiny and are sold in every artist supply store. I found them in reddish tone and in blue.
I wanted to use them to make a mold for epoxy resin, but ended up using them instead.
If you ever find a transparent two-component silicone rubber, please post in this thread.