Simple motor shaft collar coupler with nut screw insert to tighten

Optional line for a round shaft or the shaft with a cube is at the end... This version is designed for a 5mm nema 17 shaft and a 2.5mm bolt, vary as necessary, add a second bolt if you want...

I did the bolt hole tight so that the nut has to be inserted forcefully inside until it aligns perfectly with the screw hole and doesn't just slide around.