A holder for different size spools: 32 mm, 38 mm and 48 mm. Print 2 of those, use 2 (or even 4 bearings) and place it on a 8 mm shaft.
Supposed to use with 608 bearings.
Inspired by http://www.thingiverse.com/thing:80580 But this one uses less material and matches better with Felix printers
Print with the settings u like, but for best result use at least support.
Carefully insert the bearings (use a piece of wood)