"Universal Spool Holder 18-56mm II, 606ZZ/608ZZ" by cntrlV seemed like a nice design, but my spool had 60mm hole.
I made a simple parametric version in scad. I added a thin boundary above the bearing, so you need to drill it out or clean up after print, but it makes printing easier. I also made a spacer to put between the bearing and the holder so the bearing does not bind against the holder.
The uploaded stl file is for a 25-70mm holder using 608 bearing.
Update: The hole for the bearing on the uploaded stl is a tight fit (I have it set to 22mm, and 608 bearing has 22mm diameter). Depending on how well your printer is calibrated, you may want to enlarge the diameter a bit.
Update 4/16: Uploaded clip that can be used instead of a nut for quicker changes. Top side of the clip (as printed) goes toward the bearing.