http://www.instructables.com/id/SHOWER-CURTAIN-Silicone-and-Glass-Fiber-Mat This silicone rubber and fiberglass mat shower curtain will probably be good for 30 to 50 years outdoors. That's the normal guarantee on the silicone rubber, anyway. ...
You can do flat designs, or flat parts for larger designs, by extruding silicone on a flat non-stick surface. Polyethylene plastic is a good material for the flat surface. You can stretch trash bags over a board as a cheap surface to work on. I prefer a rigid polyethylene kitchen cutting board, like this one I got at Sam's Club.
After the silicone hardens, you just peel it right off.
The parts below are mostly fins for a fish sculpture I made.