I hot glue the model to a small piece of Formica. Here I am hot gluing a paper popcorn cup down around the model to contain the rubber.
This is the model, cup and wooden dowel (to make it easier to demold and to save on rubber) ready for the silicone rubber.
After mixing it completely with the spatula, the silicone mix is placed into a vacuum chamber to boil out any excess air. This picture shows the rubber mix boiling away (at room temperature).
The next morning I demold the eyepiece, and cast it in a charcoal colored urethane resin. I sand smooth both the interior and the exterior of the casting.
Now for the decoration. Rather than add screws or bolts as I have with earlier steampunk eyepieces I have decided to add a design in bas relief. First I draw out the design with a black Sharpie™ on white paper, then scan it. Bringing it into CorelDraw™ I vectorize it and clean up the lines, then laser cut it out of green card stock to check the fit onto the eyepiece.