Yes I agree: the box is intended to “float” - the proximity to the brace is just for illustration purposes.
I can always beef up the brackets a LOT, even make them a solid flange if need be, and properly triangulate them - no harm done here if it’s a problem, and I’m happy to redo anything along the way - the “mechanics” of design is something I think I understand enough to be confident experimenting with - as opposed to the electronics and code, which can only end in tears!
Oh, and my fallback position is “build it to plan” - nothing wrong with that either! 
