What you are about to see is a very rugged way to mount the boom to the servo (it can be mounted differently).
I machined two spacers which goes inside the holes of the main gear. There is a lip on the spacer so it can't fall off.
Please have a look at the 2 last pictures. That was the original way to mount the gear to the aluminium boom.
The bolts are very short, vibration got them loose. That is why I created the spacers. The very first idea was to drill and bolt directly in the gear.