[Aerodyne]

Good point, Paul. Unfortunately most of the available moulding materials are expensive to by on the retail market, and the quantities are often very small. The cheapest way is to use Alginate, I believe, for the odd couple of knobs, at least.
It is only when you have a set of value with a damaged or missing knob that cannot be replaced and has to be specially manufactured that moulding one for yourself really becomes viable in financial terms. In other terms, however, such as pleasure in creating, moulding and casting your own has more going for it.
Siligum sounds interesting, though...
-Tony