UPDATE 5: Fixed the rod models.
UPDATE 4: Added the 5 bearings version rod as requested. The same pin lock can be used.
UPDATE3: Added the 5 bearings version with smaller gap between the bearings.
UPDATE2: Added the pin suggestion to use instead of a threaded rod, and pin locks to insert next to the bearings (to lock the pin in place to avoid it collapsing).
UPDATE: Added a longer version of the spool holder SpoolHolderUpdate3.stl for larger spools.
I wanted to use 608zz bearings with the spool holder at https://www.thingiverse.com/thing:2988540 so I edited it in OpenScad. Remember to use the original file with OpenScad to make changes.
You will need the nuts from the source thing. Download from here:
I did not modify the nuts, so did not include them here.
SpoolHolderUpdate has 1mm extra space for 608zz bearings to roll freely.
SpoolHolderUpdate2 has 2mm extra space for 608zz bearings to roll freely.
You can use 8mm (5/16") dia of length ~240mm (>200mm) threaded rod to host the 608zz bearings. Since the threaded rod binds to the plastic, nuts for the rod are not necessary.
You can use 8mm (5/16") round rod as well, but you have to solve the problem of holding it in place. Maybe rubber bands or tape at the ends?