Shimming camper to minimize side to side movement

Both of my campers had metal flashing at floor perimeter, with sticking screws.
That would rub spray-on bedliner if used with no mat.
Another thing I notice on my dually was the bed ribs ending about 3" before bed end. That, when you load long camper not only can bend the ribs, but makes for longer camper overhang, lowering the stabilization.
I cut to shape a strip of 3/8" plywood and put under the mat to make solid support for camper edge.