I did 3D print a complete drive train :

It is composed of :

• a beam where everything is secured. This beam can be screwed on a chassis or integrated directly a in R.Hasika or R.Ian.
• a motor support, where the continuous servo is secured, and it then screws on the beam. Can be changed to adapt to other motors.
• two "wheels" : those are larger gearings with a lip on the side to keep the tyres in place
• two ninjaflex tyres, that have cogs matching those of the wheels, but on the inside, and external configurable sculptures for a better grip;
• one gearing behind each wheel (so 2 in total). Can be used to drive the wheels (here, only one wheel is driven);
• one cog that screws on the servo horn;
• 2 axles made with a large steel screw, with only the end tapped, a large smooth section (this is where the wheels rest) and a large cylindric head (can be seen in the center of the wheels)

Other than that, the wheels and the cogs are secured together using 4 m3 screws, the wheels axle are just screwed into the beam with the wheels in plae, and the motor is secured to the beam with 2 more m3 screws.

On this picture, the wheel assembly (without the tyre) can be seen from behind.