The nut on the side cover is what I was referring to. If you slightly overtighten it and feel binding at full lock turns, then you know you've overtightened it. At that point, back it off a bit and test again.
I have never seen a minitruck box (and I have seen many) that didn't benefit from very judicious adjusting of the box itself. Since boxes wear in the center position (since that's where 99% of your driving takes place), it's tempting to try to adjust ALL the slop out of the middle, which then...