I made this a long time ago because I thought the "wooden arm" on the Rostock Max was, well, crappy. I designed this to fit in the same slots that the wooden spool holder arm fits into. As a system of cartridges so you could easily change out spools and have them ride super smooth on bearings.
Use this with 5/16ths threaded rod, nuts and 608 "skate" bearings. (That you can often get used for free at your local skate shop. They won't be "perfect" but they will be just fine for this use.)
You can see I have quite an assortment myself.
I included a few hub sizes here here, sorry I don't use Openscad or I would add a parametric one. There are already several parametric spool hubs out there, like these: