I use a two part Siligum silicone moulding compound from HobbyCraft or similar stores. You mix the two rubbery parts and wrap it around the knob and after 5 mins it sets. The resultant mould is similar to Latex rubber.. pliable and fairly tough. It is expensive, however they say the mould can be used up to 50 times. But then again who wants 50 knobs?
Paul.