I've done this before, my motor shaft is threaded for M8 bolts so I got a piece of PVC tubing and two end caps, glue one end cap in place with epoxy or something then use a M8 bolt and bolt the PVC pipe to the shaft of the motor (to make it more secure you might want to drill through the head of the bolt and put a small pin stock through it and through the PVC as well to help it prevent not spinning,) and tape the other end shut with a cap. Just make sure to spin the motor the opposite way of the unthreading also slow as possible. Then place a set of skate wheels to support the PVC pipe and make sure to level it so it will spin evenly.