The four square gears are mounted on a plate and driven by a DC motor.  A single 2032 coin 3V battery provides the power for the motor.  C clips retain the gears on their shafts.  

Rather than need a lot of support for the base and motor support,  I made these two pieces separately and hot glued them together.   You could also use super glue.

The motor has a small concentric collar around the drive shaft.  This goes into the small hole in the base.

Three of the gears have larger holes that mate with the 6 mm shafts.  One gear has a small hole with a flat that mates with the motor drive shaft.  Line up the flat in the gear before press fitting it onto the motor shaft.  Clearance between these surfaces was .13 mm.  

Hot glue or super glue the battery holder to the base.  It has an on off switch built in.  I cut back the battery case wires to about 30 mm to reduce the excess wire needed.  The motor comes with wires, which I found to be insecure.  I soldered the battery wires to the motor directly.  

Material Needed:

Mini Micro N20 Motor, 3-6 V, geared to ~30 RPM, $7 for qty three, Amazon

2032 3V Battery, $5.35 for qty 10, Amazon

2032 Battery Case with Built-in Switch, Collao, qty 6, $5.59, Amazon

Hot Glue Gun or Super Glue

Soldering Iron with Solder and Flux