Problem with hard boxes on a off roader is everything inside gets shaken to bits unless it is strapped down / wrapped/ separated. And you cant squueeze a bit more in - it either shuts or it doest.
A rack and a bag work much better for me, , most have a pop top which can be pressed into action if required , and pouches on the outside for the slime pump etc that go on every trip.
But back to the origonal post -to get a box on my GS I just bolted a bit of black painted 45 x 200 mm lumber about 200 mm long to the GS rack,and screwed the mounting plate to that.
Then , when the stock latch got a bit loose, which didnt take long , I screwed down through the bottom of the box onto the timber to make certain the box wasnt bouncing along the road behind me.
The timber seems to dampen the vibrations out too - a mount plate which isnt backed up with something solid can vibrate into a fracture PDQ too - DAHIK