I used some pieces off a Knex kit with my mbot and made this
The tracks run around the wheel onto a black gear.
The blue bar at the back is to tension the track. I tried using less track but the wheels and front axle were bending too much.
The two black wheel things spin independently on the gray axle.
The track occasionally slips off so it needs adjusting
On surfaces such as carpet it does not turn well and i am worried about damaging the motors due to the high friction. But on tile, lino or anything smooth it works mostly well
Post if you have any questions.