A Home-made Centring ToolI made a centring tool for wheelbuilding many years ago; it works very well. Here are the details for anyone interested.
I made it out of curtain rail sections, bolted together. It's in the shape of a very flattened A. The curtain rail is the kind with a square cross-section, except for the crosspiece (supporting strut) of the A, which is a flat strip. The top of the A accommodates the adjusting bolt. I made this from a length of threaded rod, fitted with an old axle spacer at the end to fit over the end of the wheel axle. The bolt goes through a nut cemented into the crossection of the centre piece. There's another nut at the top of the rod and a weak compression spring to keep it taut. I bevelled off the feet of the A and fitted short sections of curtain rail to each foot. The only thing I needed to buy was the threaded rod. You could use a long bolt if you have one.