This Thing is a modified version of jsirgado's caster, with wheels of my own design, to fit my Broyhill model 41119 office chair (well, that's the model number used by "True Innovations", the supplier; it's actually a "Wal*Mart fall-apart special" :P ).
To be specific, I shrunk down the axle "hump" so that the caster fits the chair's ~50 x 5.7 mm wheel axle, and ~7mm diameter caster pivot (that's the lower part, 10.9 mm in the upper part). Grab a hammer, you'll need to pound pretty smartly to get the pivot and axle into their respective holes in the caster and wheels. :-) You may also need to pass a drill bit through the caster's axle hole to clean it up (easily done by hand).
The wheel is closely based on the chair's original wheels.
Use supports for both; beware of supports being inserted into the bottom of the caster pivot hole.
Someone in another Thing suggested printing their version with 2-3 mm worth of shells/top/bottom and high infill. In my case, 10 top/bottom layers and 8 shells.