The shaft on a Tetrix or Andymark NeverRest motor is 6mm. So need to get from 6mm to 1/2" hex for the Mecanum wheels you mentioned. Since the wheels are so wide (2.25"), I would recommend the really cool
ThunderHex shafts from Vex Robotics. You could cut to the desired length, enlarge one end with a 6mm drill bit to fit on the motor, then drill/tap two holes for set screw. The wheels can be held on the shaft with Vex Robotics 1/2 hex
clamping collars.
We built some shafts and drill/tapped with 8/32 and used the HeliCoil inserts to make sure they were strong. Never had a problem all season! Here is a
link to the HeliCoil inserts we purchased on Amazon.