Small plastic rollers can be handy - better than having your latest robot dragging its butt across the table! These Pololu units are inexpensive, small, and easy to use!
These all-plastic casters are small, light, but functional! If you're looking for a 3rd touch point on your small mobile device, check these out. Drill a 0.39" hole (25/64", or a 9.9mm, but we're pretty sure a 3/8" hole would work fine), and push fit this in.
It has a 11.5mm (0.45") stand-off, with an overall length of 20.5mm (0.81"), and a 12mm roller-retainer sleeve diameter.