Suggestion: cotton balls may shed lint or fibers which won't look good stuck in your paint.
You may be better off using multiple notched sheets of paper. Slip the papers around the perimeter of the knob, with the knob shaft in the notch.
Or you can try masking tape: multiple strips of masking tape, sticky side on the radio face, non-sticky side up under the knob, using multiple strips of tape to surround the knob shaft.
In either case use lots of newspaper around the prepped knob to protect the car interior from overspray.