Here's my extendable boom mod:
folded:
It took me quite a while to figure something out that was able to fold completely. The trick was to place the piston on the side instead of on top.
I was able to route the extra hoses trough the turntable, but turning the crane now needs quite some force. I even had to reinforce the gearing because they would sometimes skip. This can probably be improved with more careful routing of the hoses.
A third pneumatic switch is placed in between the existing ones.
I'm quite happy how it turned out, it doesn't look as bulky as I feared it would.
On the downside, the piston doesn't have much force and sometimes struggles to move even without extra weight on the hook.
(P.S. this is my first real post on this forum, even though my account is 10 years old this month! please be gentle :-D)