Thanks guys, for the nice words. And Bob, you are too kind!
Andy, I use silicone RTV from Alumilite, HS-11.
https://www.alumilite.com/ProdDetail...th%202%20(HSII)
But you can get silicone rubber from many places. This particular version mixes 10:1 by weight, but you can get types that mix 1:1 by volume as well. The HS-II is stretches a lot and resists tearing.
Now, as for the plastic, Smooth-On makes a bewildering variety of liquid 2-part plastic products.
http://smoothon.com/liqplas.htm
I choose the Smooth-Cast 322 product, which cures to a off-white opaque color, can be tinted to any color using pigments (from Smooth-On), has a pot life of 20 minutes, and does not seem to be very moisture sensitive. They also have clear products, black products, and a wide variety of pot lives and hardness. Some types are very moisture sensitive, so read the product data sheets, and if you call Smooth-On, they can advise you on porduct choices.
The stuff is not cheap, a 1 pound ration of the Silicone RTV is about $30, and the plastic is about the same as I recall.