ways of securing the knob to the D-shaped axle

Didn't notice this when you posted initially. Are you saying that your unit has its knob attached to an axle that is not wholly cylindrical? That it has a flat face molded out of the curved surface, so that it's capital-D-shaped?

If so, my unit's not like that. I have a solid cylinder that the knob fits on. Maybe this was changed between Mk2 and Mk2a, as there was a problem with the original knobs slipping on the axle. I and (I assume) many others had to have new knobs shipped out. They fixed the problem by using a small rubbery O-ring in the new knob to grab the axle.

So either whoever needs to watch out and make sure that any newly fabricated knobs will fit on both axles, if there are two, or maybe the O-ring is your answer.