Flat filament spool roller

I got annoyed by a couple of damaged or warped filament spools I have that wouldn't work with my [other](http://www.thingiverse.com/thing:307228) various [rollers](http://www.thingiverse.com/thing:321854), so I designed this instead. Uses plain skateboard/inline bearings (608) and is sized for the spools I have around, but can be altered for other bearings and dimensions. I made the "hub" fit the smallest-diameter spool I had (about 15mm diameter center hole), and then did spacers to fit other d