I have some spools of filament that are a bit too wide and would get hung up on the nut. I also didn't like the spool riding directly on the rod. I created a rod that is longer, added bigger nuts and a spacer/bushing to take up the gap between the roll and the rod.
Be sure to print 2 nuts and 2 spacers.
Different size spacers to accommodate different brands of filament.
UPDATE: I wanted to have a way to easily see what size spacer I was grabbing, so I modeled the size into the side.
UPDATE: Added a new version of the shaft. This one has more threads on one side to better handle narrower filament spools.