I bought an old (1950's?) BK-50 Swallow Bug Morse key which needed some TLC. The original knob and paddle were both missing, so I had a go at 3D printing my own. I had to go by photos on the Web and judge size, but it seems pretty close. Printed in black PLA with 100% infill. the centre hole is 3mm, which should be the correct size for an attachment screw/stud. I cut the original screw's head off and it self tapped nicely into the hole.
Please see my other design for the matching paddle.
This thing was made with Tinkercad. Edit it online https://www.tinkercad.com/things/fFQyAmoYy4E