Thanks so far for the replies. I don't mind drilling the box but difficult sometimes to get all the holes lined up correctly. Hadn't really thought about cabling or chaining the box--I actually like that idea, it would make it easier to remove the box and still have the carrier available for other purposes. Bolting the box to the carrier essentially would make it one unit that would have to be removed at the hitch. So maybe it's off to the hardware store to see what's available.
And re Kayteg's point about engine cooling, there's about 20" between the back of the box and the grill. The truck is a Ram 3500 with snowplowing/towing 'heavy duty' engine cooling, so I suspect it should probably be okay.
Hitch pin-wise, I've got some anti-rattle hitch locks which should keep everything nice and tight.
