HLV-H stub shaft -- making a blind keyway

I have a metric HLV-H and despite being of an age where I was told the imperial system was never going to be used again find myself frequently
wishing to be able cut imperial threads in sizes by a more certain method than manual thread chasers!
Fortunately I have the 127t gear and ZMT were able to supply me with the appropriate quadrant.
However, the stub shaft which goes on the end of the output shaft to mount the 127t gear seems unavailable. By some miracle I was able to borrow one from an HLV-H five minutes drive away with the plan of making a copy.
The problem is that the piece which goes onto the output shaft has a blind keyway which I have no idea how to replicate.
At the moment my best plan seems to be to make it in two parts (with an open keyway and braze together). Previous experience cutting keyways has involved abusing an (old) lathe racking the cariage back and forth with a parting tool set sideways.....(I have no shaper or slotting) head.
Since there are a good few HLVs out there I wondered if anyone had been here before me...
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