i am in the middle of a knob making project so i can be of help...
make a square out of plaster, clay, resin, anything hard but carveable but not grainy so you can get detail.
get some rubber type liquid that dries, i think they make it for dipping wrenches in, but you can also get some at an art supply store.
coat your hand carved knob with it, make a alot of layers and let them dry in between, so you end up with about a 1/2" thick rubber cast, peel this off your hand carved knob.
turn the cast upside down and make a little jig so it holds it flat, get some clear casting resin (found at an art supply store) fill it up, dont let it kick (dry) to much cause it will get hot and limit the amout of life of your rubber cast.
hand sand then primer, sand again, then paint, and decorate with lines, grooves, ect.
sounds more complicated than it is and when your done youll have custom made knobs that are rock hard and smooth, expect to get about 8-20 knobs out of each rubber mask.
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