For the nut to loosen, it would seem that the screws must have as well. If that is the case, you could get yourself a small bottle/tube of blue thread-lock compound. I suggest the blue verses the red since the blue is not 'permanent'. Once you apply the red stuff and it sets, it is a bear to ever remove. The blue stuff stays slightly rubbery and while it holds, you can still remove the screw when the time comes. Seems like a poor design.