The case is a CD spindle holder. A thin piece -- .25" --- of model aircraft plywood was shaped and mounted to the bottom of the spindle case with plastic/wood adhesive and bonder. This "stiffened" the base so it wasn't too mushy.
The motors are Solarbotics GM8 with high-speed motors added. The ball caster is from Pololu -- but it's fragile and doesn't roll well and I don't recommend using it.
One note -- the "clear" plastic of the spindle case is not infrared transparent, rather, it is translucent to IR. The detector works fine receiving IR through the plastic, but the LEDs must be mounted so they shine outside the clear plastic, otherwise they will reflect back to the detector and create false positives during obstacle avoidance routines.
The single metal brace stiffens the center column and was machined out of .032 aluminum sheet.