I just discovered after less than a couple hundred miles total riding that the hold-down wingnuts that came with the box were loose. I know what you may think, but pretty sure I tighten at about 7 foot pounds as that size bolt requires. But with the nylon washer I guess it loosened and while rattling down the trail it rubbed off the threads on one bolt and would not retighten. So I installed graded bolts with some flat and lock washers which should keep them from loosening again. You could also add a couple nuts on the bolt ends underneath the bed, if you wanted to use a little longer bolt, fat hands may struggle to get the nuts up there but its not too hard lol.
I would hate to discover that the bolts had loosened all the way up if I turn it over and had a jail break of the box all over, so thought I would share this tidbit.