Was there ever a box built that when it was sat down load taken of the cable it would release so when it was picked up agin it would turn up side down and dump? I have cut the bottom out of a old pickup bed fuel tank that i want to fill with scrap iron so i can lift it with my winch truck set it on the trailer and have it dump.
Boscaro sells something like what you are talking about. They do it with a swinging overhead bar.
Less elaborate, more robust