I realized that hadn’t posted about the installation of the main box latch plate…
This requires a (hardened steel) latch plate be mounted to the underside of the main box.
It is partially mortised to allow the latch bolt to have adequate purchase.
Locating it is fairly straightforward, I ran a pencil along the edge of the bolt to deposit graphite onto it. Then, operated it several times (in an attempt) to leave a mark on the bottom panel. Fortunately, working the main box back and forth also left a large scratch.