I use a Reagan banjo capo on my nylon-strung Harmony banjo. It’s a little chunk of brass with a saw-cut to take the string, a thumb-screw to keep it tight, and some felt stuck on the bottom to protect the fretboard.
As supplied, the string slot was too narrow to take nylon strings, and too low to effectively fret the string. What I did was:
- Open out the slot with the patient application of fine sandpaper. This took a long time, and kept gouging up the edge of the sandpaper sheet.
- Replace the felt with a thicker piece of neoprene cut from an old mouse mat.
The capo still bobs about, but stays on the neck where it should. I still haven’t solved the problem of where to store it, though.