The rover is constructed in 3 main sections. Details of how these are built and fit together are shown below:
The Base / Drive Section connects the motors via gears to the wheels and caterpillar tracks to drive the rover. The main steps of the construction are shown in the photos below. Both sides are constructed in a similar way, with the motor driving one of the axles on each side. The other axle on each side spins freely as it will be connected to the drive wheel by the track. The two sides are connected by the joining bumper pieces and the grey plate underneath the motors. The wheels and tracks are then attached and the wires for the motors hidden inside.
The Middle / Power Section holds the batteries to power the rover and supports the upper part. The construction is shown in the photos below. The battery tray sits on top of the base section and supports the battery pack. The wires for the battery power and lights are then attached and the battery box sits on top of the tray.
The Upper / Control Section houses the Raspberry Pi, breadboard and electronics to control the rover. The upper section sits on top of the battery box and attaches to the supports on either side with the two black pins. These can easily be removed to take it off. The lights fit into the mounts on the front of the upper section. The breadboard and Raspberry Pi fit in the slots on top of the upper section. The terminal blocks for the battery power, two motor outputs and Pi output power are connected to the board and the micro USB power connector to the Pi. The outer housing parts then fit over the electronics to protect it from knocks and hold it all in place.
That's it for the rover construction. I came up with the design by several hours of trial and error, adding bits and seeing what worked. If you are trying to build your own version, hopefully you can adapt this design to use whatever parts you have. In the next section I will explain the electronics to get the rover moving.