Nice one! They came out well, and certainly look the part on the radio (nice radio by the way!). Bubbles in the mould and the final piece are always a problem, eliminating them totally would need a vacuum chamber, which is probably out of reach of most of us restorers, but making the mould with the knob face up keeps the worst bubbles away from the face of the knob, and then turning the mould upside down to cast the final piece makes sure the bubbles end up at the back of the knob out of sight! I'm guessing this is how you made yours too
Regards,
Lloyd