The InnerTrackFrame – DCMotor is a block that fits into the InnerTrackFrame Mounting point.
It is design to support the common no name yellow dc geared motor. You can find them here.
NOTE: You can find the main page of the project, with the index of related links and articles here.
The design tries to use the least amount of plastic possible, while being sturdy enough to handle the load of the rest of the rover.
You would need to print four of them, one per motor, in case you are using four motors, as it is usually done on any smart car powered with this kind of small yellow no named geared dc motors.
On the following video you can watch this step by step how to assemble the InnerTrackFrame – DCMotor blocks with the rest of the blocks.