I have created a wheel design for driving the tank tracks for my mini Raspberry PI robot. The STL file is available here on tinkercad.com.
Also found a couple worm gear motors that will run at 30rpm at 12V (Unloaded) on Amazon.ca . They have a 6mm D-shaft that is a little short, but should be ok.
Next step is to build a 3D design for mounting the motors and holding the wheels in position within the track.
Will try to post some pictures soon.