I got annoyed by a couple of damaged or warped filament spools I have that wouldn't work with my other various rollers, 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 dimensions.

Uses pins.scad from this library to hold the bearings: http://www.thingiverse.com/thing:10541

Print a "hub" part with the center diameter to match the smallest spool you have. Attach bearings to the pins, they may require some filing down to fit depending on your printer (it is fairly tight). You may need a track for it to run on, depending on the surface.
Just print spacers for whatever various spool diameters you have (or just more "hubs" with different sizes). For spacers smaller than 30mm (or less than 10-15mm more than the hub diameter), the "small" spacer (solid, no spokes) makes more sense.