Either a fifth string capo or "spikes" (small tacks used by model railway enthusiasts for holding down train tracks) which you can tuck the 5th string under.

Both of these are fine (I prefer spikes as I find a 5th string capo on a slider gets in the way) but it is still difficult to change keys during a song or tune set.

You always end up having to fine tune when you apply a main capo or a 5th string device as banjos seem more sensitive than g*itars. More of a precision instrument maybe.....

Or is it that g*itarists just don't care?