I tweaked MarkBenson's design and took a stab at creating my first Customizer.
Use the Customizer to change wheel diameter, remove the spokes (customize wheel in Tinkercad!), add text, create a test-fit piece, or tweak the tolerances for a better fit.
Use a common O-ring for traction.
Full drawing robot project documented at http://www.instructables.com/id/Low-Cost-Arduino-Compatible-Drawing-Robot/
If your printer isn't calibrated (feed rate, etc.), your hub cut-out may need to be enlarged. My printer, for example, needed to add about 0.5 mm to cutouts before I calibrated it. See http://www.thingiverse.com/thing:342198 for calibration information.
I've learned to save some material (and time) by printing out test pieces first.
You can use a 3mm set screw if needed. You may need to tap it.
Prints without support. I would use 100% fill for strength around the hub.
For the 60mm version, I used a 1-7/8" ID x 1/8" O-ring from McMaster-Carr: http://www.mcmaster.com/#9452k96/=zksola.