The Kyser KBMBA Quick-Change that's specifically made for the necks of smaller instruments like the banjo.

Whether it's for bluegrass, folk, Americana, country, or rock, the banjo is once again gaining popularity. This Kyser® Quick-Change® is expertly engineered and built to last. It is reliable and does just what it is designed to do - clearly raise the pitch of the banjo so you can play in a different key without retuning or changing fingering. Seasoned professionals and beginning players alike appreciate how the Kyser® Quick-Change® helps them transpose their banjo in seconds, with only one hand. The beauty of the Kyser® Quick-Change® is in its simplicity. We pioneered this design and although often imitated, a Kyser® is never equaled. There is no substitute for a Kyser® Quick-Change®.

With a bit of practice you could even change key mid-song!