My initial design used just 3d printed PLA to act as a concrete mold.   While this was a good Idea I soon found out that Concrete and Grout Stick to everything and are difficult to separate.   To remedy this I decided to go with making a silicone master mold.   

To do this I start out with making a PLA version of the Knob with a PLA Ring to act as a mold box

Once printed I spray painted using several coats of High Fill Primer and sand the Knob down for a smooth finish.

I then poured Moldstar 30  Silicone into the cast.    I waited 2 days for a full cure. 

Once demoted simply pour concrete and wait 1 day for concrete to cure.