Middle plate design

The dimension of this plate is same as the base plate and the screw size is also similar:

Design of middle plate of robot

The middle plate can be held above the hollow tubes from the base plate. This arrangement is connected using another hollow tube that extends from the middle plate. The tube from the middle plate will have a screw at the bottom to fix the tube from the base plate and the middle plate and a hollow end to connect the top plate. The top and side view of the tube extending from the middle plate is shown in the following diagram:

Design of hollow tube 20 cm

This tube will connect the middle plate to the base plate and at the same time provide a connect the top plate.