Unless you want the ability to change handles, another option, if you have access to a lathe, is to anneal a large steel ball bearing, grind existing bolt knob down and drill hole in ball to fit the stub. Soft solder ball to handle. IIRC, this is a 3/4" ball which I put on a 1903 target rifle bolt.
Flatten bottom of ball & stipple to provide room for trigger finger with the larger ball.
Regards,
hm
