Parts-Description and Building Instructions:

Base-Board:

4mm Fiberboard, foam board or any suitable material. Make sure it is not too thick; the wheel axle mount needs to fit under it.

"Third wheel" - the glider in the front

I recommend a cut-in-half ping pong ball. I know, All pictures here show another plastic-thing, but I did not have ping pong balls at hand originally.

Drive unit: 

All 3D-models are available here:

https://www.printables.com/model/915546-simple-robot-car-wheel-drive-train-gearbox

Drive unit hints:

• The axle holes in my model are adapted to my 2.5mm axles and the quirks of my 3D-printer. For precise hole sizes, you might want to 3D-print a hole size template first, with the correct hole sizes in a flat piece that prints quickly. It should have:

• • A hole that gives a good sliding fit for the axles (for the 2 bearings)
  • A hole that is a press fit on the axles (for all gears)
  • A hole that is a screw thread press fit (for the M4 wheel bolts)
• All additional parts can be seen on the "explosion" view left. The steel axles can be made from coat-hanger wire, but then you need to check what diameter yours is, and probably adapt the 3D-model accordingly (see last step)

• All holes in the base plate can be drilled 2.5mm, and the M3-screws can simply cut their own thread. Does not work with foam board, i guess :)

• The drive units can be used as drilling template so that the holes in the base plate are precisely where they need to be.

Electronics hints:

• I used green LEDs because they are very bright at 3V. The LEDs were SMD, which is why the LED/sensor assembly is a bit complicated.

• Technically any photo transistor should work. For photo resistors, some tests are needed.

• There is no sensitivity finetuning on the electronics side (yet). It is possible that the vehicle does not work because the sensor might be too sensitive. Bending the sensor units a bit away from the ground helps usually.