I used a leather strap. Bought some thin leather from Hobby Lobby. Cut a long strip of correct with a razor blade. Pre-stretch it then cut the strip a slightly longer than needed. Then with the razor blade cut taper (sloping) ends and then lap the ends together with a dab of white glue or super glue and clamp until glue is hard with a pair of pliers. Belt still working fine on my KRK5 tuners (three of them) with no slip after 20 years.
The same method was used for the mandrel drive belts on the earliest Edison cylinder phonographs from 1888 thru 1912 except they used fish glue. ( I have a 1900 Columbia AT, 1901 and 1903 Edison Model A Standards, a 1906 Edison Model B Triumph and a 1910 Edison Fireside which have belts made this way and play faultlessly).
Here is a YouTube video of the process.
https://youtu.be/NF9A-_xMBck?si=3yBpeNXUjuekhMDD