The original knobs are 3 or 4 pieces. There is a plastic threaded knob, a plastic icon part, an outer rubber, and some have a plastic light pipe.
I just made the rubber part that slips over the original plastic parts. The rubber is 50 Duro EPDM.
The mold is CNC machined out of aluminum, with a removeable aluminum core. You can see from the cross section that there is heavy undercuts in the part. This makes it tricky to get the part off the core.