If you want a more layers for the robot, you can simply make circular plates and hollow tubes which are compatible with the base plate. Here you can see middle plate design and the tubes used to connect it to the base plate:
Figure 4: The middle plate design
The middle plate is simply a circular plate having screw holes to connect it to the tubes from the base plate. We can use following kind of hollow tubes to connect the base plate tubes and middle plate.
Figure 5: The hollow tube from the second plate
Here you can see that a screw is mounted on one side of the tube; the screw can be used to connect tubes to the base plate. We can mount the top plate on top of the tube too.