Remix of http://www.thingiverse.com/thing:286000
These files have been optimized to work with a Rep2 printer, printing at standard resolution, no raft or support.
For the ball bearing I use a standard mouse ball as seen in the picture. With the rubber on, they are about 7/8"/22mm. With the rubber removed they are 3/4"/19.02mm.
This makes a caster just slightly bigger than the OEM caster ~0.025"/0.7mm
Special thanks to projunk http://www.thingiverse.com/projunk/designs for the original design

These files have been optimized to work with a Rep2 printer, printing at standard resolution, no raft or support. Holes have correct spacing but may require drilling to remove imperfections. I used a 3/16" drill bit.
For the ball bearing I use a standard mouse ball as seen in the picture. Be very careful when cutting the rubber and never cut toward yourself. With the rubber on, they are about 7/8"/22mm. With the rubber removed they are 3/4"/19.02mm. Alternately you could buy some 3/4" ball bearings.
This makes a caster just slightly bigger than the OEM caster ~0.025"/0.7mm