So, I have these invisible knobs...

[amptramp]

The antique radio world has embraced 3-D printing as the way to replicate knobs that have vanished into oblivion.  Quite often, a radio is found complete except for the knobs and if you know what they look like, you can rebuild them as they originally were.  The last time I bought knobs for a specific radio, it was a Philco model 70 and the four knobs cost $45 CDN.  Lots of incentive for making your own.

For metal knobs, you can make a mould around an existing knob in plaster and replicate it in aluminum or some other moderate melting-point metal.  You could produce a number of them from one mould, so you could go into production.