To make sure that the lever arms find the right way into the buckle and stabilise the complete arm you need a guide tube.
I placed the servo in the middle of the book with double side tape.
I used something a brass part from an old wall clock as a guide tube.
I also drilled it up to a little over 2.0mm and make sure that the 2mm pipe moves smooth through but is not too lose.
I file a small groove into the guide tube get a better connection with the book cover.
Then I glued it with second glue on the cover.
Then I build the level arms.
This was a really hard job because of the joint which I build myself. Maybe it is easier to buy in a model store a small knuckle joint for an engine and use the two single joints.
I cut a 3mm long 2.5mm wide slot into a 1cm long 4mm brass pipe.
Then I drill a 1mm hole in a 90 degree angel to the slot.
I stick a 1mm brass nail through both holes.
Make sure that you can turn the nail inside the slot without any resistant.
Then solder the 2mm Brass pipe on the 1mm Brass nail.
That is the most complicated work on this project!!!
Only get solder on the connection between nail and 2mm brass pipe if some solder stick into your 1mm hole and nail you failed. And the joint don’t move in any direction.
I need 6 to 8 times until I only solder the brass pipe it will help a little if you put some acrylic paint in the connection between nail and hole. It’s like a cover on the brass and prevent that you solder flow into the hole.
If you built this joint nearly all your work is done.
The last step is to stick a short 1mm brass bar into the 2mm brass pipe and figure out the right length to the servo adjust it in a position that the lock is completely open but don’t move out of the buckle adapter! I also use my solder iron to fix my bar into the right position.