I bought a 3 conductor stereo cable with 3.5mm male connectors on both ends.
Chopped off one of the connectors, and attached crimp on terminals.
Secured those with a drop of solder, and a little bit of shrink tubing.
The crimp on terminals attached cleanly to the screw terminals on the underside of the BY-1 base.
The 3 conductor 3.5mm plug on the remaining end is exactly what I needed for my
Yaesu FT-817, FT-857D, and FT-897D. Works great.
You might have to move the connections around a bit to get the dit paddle and dah paddle in the right spot.