I did my mog wheels by putting them on a axle and rotating them, with a run-out gauge.
Very awkward to do, and get right that way.
The way (I think it was Paul W suggested) is to get a flat bench, put a large (10"+) tube through the middle of the rim.
The tube is cut to the height where you want the wheel centre.
You can just then put the wheel centre in, tap it down so it rests on the top of the tube.
Providing, bench is flat and the tube is cut accurately - one square rim ready to weld up.
Actually thinking about it, if you were to do it up-side down, you could then put the tack welds into place without disturbing the "jig"
Clear as mud