Mines mounted on Yakima cross bars about 1 1/2 inches above the edge.
I like it mounted lower even with the spring over it sits high above the ground.
I also have a tailgate cut so that makes it easier to remove camping gear from underneath. I place my water & fuel cans at the front the way they load they cant be removed with out pulling more gear from the rear so someone cant pull things out of the trailer with it mounted higher things can be removed easier but things can be stolen easier too. A lid would take care of that problem too.