Actually Self-Centering ReFiller Spool Roller

Who doesn't love a good frictionless filament spool roller? I originally printed the classic [Filler](https://www.thingiverse.com/thing:3020026) and it worked very nicely. The only flaw it had was that the spool would eventually slide over to the side and sometimes hop over the rim and start scraping the bearing. So I found ReFiller, an alternative roller that promised to be self-centering. However, after printing it, I discovered it was actually the opposite. Spools would actively ride up the edges.

After some thinking and some physics lessons, I discovered that counterintuitively the roller needs to be convex, not concave. So I got to modeling, printed it out, and what do you know, it works!

Print two copies of the roller and superglue them together. Stick two 608 bearings on the ends and you're good to go! I've included jonspaceharper's original holder for convenience, though it's unchanged from his thing so if you already have his you don't need to reprint it. However it's di