Pulley / Idler measuring jig

This is a jig to measure the concentricity of a pulley / idler you could also use it to measure if the axle of a stepper motor is bent. It is designed on this dial indicator:

https://www.ebay.de/itm/STEINLE-Me%C3%9Fuhr-Messuhr-0-10-0-01-mm-mit-Metallge%C3%A4use-und-Toleranzmarken-3105/172253576787?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

and is supposed to be mounted on a piece of 20x20 extrusion.

You also need a looped gt2 belt. I got one with 200mm length but you cou