Obtain the following parts:
1) Gear Motor - Pololu part number 2366 (http://www.pololu.com/product/2366).
2) Battery Holder - Radio Shack part number 2700401.
3) Switch - Radio Shack part number 275-0624.
4) 4 9.5mm (3/8") ball bearings - I found mine at Ace Hardware.
5) Rubber band, 1mm thick by 4mm wide.
Print the parts.
Test fit all parts before assembly.
Break off the hold down disks from "Base.stl" and "Top.stl".
The drive wheel assembly is similar to a car wheel / tire assembly where "Wheel Motor.stl" is the wheel, and the rubber band is the tire. Start by cutting open the rubber band such that after the cut it is a strip. Place a small dot of thick cyanoacrylate on "Wheel Motor.stl". Press one end of the rubber band onto the dot and hold until it sets. Roll the rubber band smoothly around "Wheel Motor.stl" until it meets the beginning, then cut it to fit. Place another small dot of thick cyanoacrylate on "Wheel Motor.stl" where the end meets the beginning, then press the end in place and hold until until it sets.
Press the drive wheel assembly onto the gear motor shaft.
Solder a wire between the motor "-" terminal to either terminal on the switch. Solder the battery holder black wire to the remaining terminal on the switch. Solder the battery holder red wire to the motor "+" terminal.
Install the motor assembly and switch in "Base.stl".
Install the battery holder in "Top.stl".
Install the top assembly in "Base.stl", carefully routing the battery holder wires down each side of the motor then towards the rear of the base.
Test fit "Axle.stl" in "Wheel.stl", making sure the wheel rolls smoothly on the axle. If not, use a 5/32" drill bit (in reverse), file, and/or modeling knife to carefully enlarge the hole in "Wheel.stl" until it does.
Place "Axel.stl" in "Wheel.stl", then press into "Top.stl".
Place ball bearings in the geometric figure tracks.
Turn it on.