Hi Joe, glad to have you here, and thanks for the question. We've done a video that might be helpful. I will link it down below but essentially what you can do is run a 3/8-16 threaded rod through conduit and use some irrigation PE or PEX tubing as a shim around the threaded rod. Then you'll just use some washers and nuts to hold the wheels in place.
Another community member named Aksel has a great video where he explains it with the PE tubing instead of the PEX.