The problem starts from the first boom extension and ends in the second boom extension.
See? Gap and gap. The second boom extension, composed of Technic studded beams and smoothing tiles, will go wiggling about inside the frame I used for the first extension. How can I make a flush yet smooth first extension to fit the beam, while at the same time still be able to fit smoothly and flush to the main boom? I can use advice from expert crane makers. HINT: the completed crane is 8-wide.