the locking element was where the fail condition was in the other ones, it wound up being just a few layers and structurally unsound, probably a design constraint because otherwise the walls were too thick.
As an Individual I am not excited about locking stacking elements because its like another element to worry about if you have to get to a lower layer. When its ruggedized and in a penguin case it's a whole different vibe but I am ambivalent about locking for things that get fiddled with personally. Both the mock ups and the orange prototype you designed are beautiful btw.