Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4983456.
Replacement screw insert to support M8 threaded rod-based universal spool holders. It is essentially just a cut and rejoin of the spool holder part from wwendorf's thing (below).
Uses frame mount from: Filament Spool Holder for Railcore II Just replace the spool holder part with it and insert M8 threaded rod.
I used the following additional things for mine but it should easily work with anything that's designed around an M8 rod mount.
Spool Holder: Parametric universal spool holder
Wingnut (for convenience): Instant Wingnut Adapter M3 M4 M5 M6 M8 M10
Additional parts: 3 x M8 hex nuts (one for each end and one as a spacer with washers on either side between the frame mount and inside hub bearing) 4 x M8 washers