The license plate stash boxes look perfect except I can only find plastic ones. I might be able to mount a steel pistol case in the same manner but I'd have to fabricate a new license plate bracket because the one on my bike is plastic and not very durable. I wouldn't want too much weight on the stock bracket.
Behind the license plate appears to be the most discreet position possible. Also it sounds most people use them to store registration, insurance, and small tools like tire gauges, so perhaps the chances of someone noticing such a box and wanting to break into it are very small.
This should be an interesting project. If I proceed, I need to make a steel bracket, securely mount my pistol case to it, reasonably weatherproof it, then mount my license plate on top.