I can almost get the desired behavior just by laying the cover on the top of the stuff that the box is intended to cover (e.g., PCB's, 66-blocks, etc.). But, cables (service loops) end up pushing out on the cover so it either doesn't lie perfectly "flat" (granted, that's just a cosmetic issue) *or* it falls off easily (with any vibration to the wall).
But, this only (partially) works on the wall-mounted boxes. I have at least one that is ceiling mounted (i.e., box has open face pointing skyward) so it needs to be held from "all (many) sides".
Somebody makes a "stud" that terminates in a ball/sphere. This mates with a spring-formed "catch". Something SIMILAR to: or: or:
These could work. Put the ball parts on the inside of the cover and the "catches" recessed in the "board". Then, add some sort of guide that makes it easy for you to line up the four "studs" without risk of mashing wires or components (most of these things are in hard to access locations so expecting the user to be able to clearly *see* what he's doing would be presumptuous)
Another approach might be to fasten on the *outside* of the box. I.e., a lip on the board that holds the "open" face of the box in the correct location. Then, clamps that slip over the top/bottom or left/right edges and pull the box *to* the board.
Sort of like the way a rotor cap was held onto a distributor...