I just re read this. A SMALL washer. Smaller than the OE washer I assume. Thus that side of the bolt and still slide in the slot, and the SMALL washer does not contact the camming tabs on the mount. Not ideal but I guess it will work.Just a small washer a little bigger than the diameter of the nut. I marked the positions with a paint pen after I re-torqued them and I haven't noticed any movement in them after any of my trips.
That is why you said it doesn't move.