Small change since the last update: I will not use this old wooden box found at the flea market, eventually, although the mechanism was originally designed to fit inside.
It turned out that it was not dissuasive enough: it was much too easy and tempting to make a small, discreet hole to operate the opening mechanism.
One can not imagine what an honest man can do when something is missing.As Charlie suggested, I finally made it with 10mm Plexiglas.
It is already machined, but I still have to glue it.
I will put the real pictures in a few days.