Looks like a very useful box. The knob on the top looks a bit overkill though, you might have been better with a small "hasp" which would enable you to use a padlock. I don`t suppose there is any law against having a big knob.At one time I had a large box on my bike for shopping. I built it from "Corex" which is the stuff used for estate agents signs. It is very light and quite strong , and you can fix it with cable ties, and a hot glue gun. Mine was in the shape of a combined top box and panniers, and could hold loads of shopping .
I had a sign on the back of mine, saying "Caution, Doddering old git". Talking of matters legal, I keep thinking of putting a number plate on the back of my Ebike, just for a laugh, with a made up personalised number. I wonder what the local plodd would make of that?