The original wheels used a 12mm hex nut fastened to the wheel with an M4 bolt.

I expected to be able to use the existing 12mm hex nut

A suitable set were found on Amazon (for a reasonable price).

The mecanum wheels required a longer hex nut as they didn’t provide sufficient clearance.

Amazon came to the rescue with a 24mm version which provided a reasonable clearance.

This is the robot with the mecanum wheels on the extended hex nuts.