This is a very simple spool holder. It works a treat :)
I printed main side pieces at 30% infill (for extra strength). the rest at 15% infill
It uses standard F688ZZ bearings that are 16mm OD and 8mm ID - they actually have a flange that stops them being pushed all the way through but it is not required at all, any 16mm OD and 8mm ID will work.
once printed you can glue the roller parts together - mine isn't it's a great fit.
it clips on to the standard PRUSA i3 mk3 aluminium frame.
i only printed the side pieces at 30% infill for extra confidence - i'm not sure it's needed :)