square hole, round shaft mate

Two "width" mades will do the trick.

As an alternative, I would draw a diagonal centerline acros the square hole. Then I would add a sketch-point at the midpoint of the centerline I just drew. After closing out of the sketch, I would go to insert>reference geometry>axis, and make an axis with that sketch-point and the plane that was used as the extrusion plane for the square

Then I would mate the shaft concntric to the new axis.

This is a little more robust if the width if the square hole or the shaft changes during design changes Ed