Shaft coupler question

[?] What is the best coupler for 28BYJ-48 ?
The shaft is half rounded and the following coupler doesn't work.

Any help?

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A shaft coupler that fits the outer diameter of the flatted shaft and a setscrew to engage the flat will work.

The coupler DOES work all you have to do is rotate so that both screws are on the shaft OR so that just one screw goes onto the flat.

No problems here with those.