OP Long gone But for the other readers , upright trikes I put together all have a steel keyway Shaft driving 1 wheel,

the final drive threaded freewheel mount has a opposing pair of setscrew, 1 bites into the piece of keystock.

Thanks to the China based low cost manufacturing, the freewheel Mount thing does not cost Much

If you have a Lathe and can chuck the piece in It .. the tool bit to buy fir the tool holder piece is cheaper.
To cut an outside thread the spinning piece (with the right feed rate)..
but having a lathe you wouldn't have come here to ask and probably be done by now..