Banjos can use a regular guitar capo but there is a problem if you want to use it below the fifth fret on a five string banjo because the G string next to the thumb doesn’t start until that point. The solution is to either drive spikes into the neck which the high string can be stretched round or, for a less permanent and more flexible solution, to use some kind of device to hold down just that string in a different position.
A cheap solution is to use a sawn-off plastic lid from a cheap ballpoint pen. I’ve done that for a couple of years but it does have a tendency to ping off at inopportune times. A few days ago I took a look at some 3D printed options. Unfortunately one was a bit low for the action on my instrument and came off too readily. The other was a bit higher and seems okay with the limited testing I’ve done but it does leave the string with a bit of a plinky tone and still doesn’t entirely inspire me with confidence about how well it will stay on.
I think I might see if I can design some improvements. For example, there could be more support behind the fret and perhaps even above it. That short string is rarely fretted so extra material probably wouldn’t get in the way and it might add to stability. Another thought is to mount a thin rail between the tuning peg and the body. That way a little peg could be used to pick a fret and it would have extra support from the rail as well as using the pressure of the string. Actually, doing some further research, that seems to be essentially what the device known as a 5th string capo is, so my design thinking might end up being along tried and test lines.
