Taking the "slot" idea a bit further,a series of recesses on the nut side the same depth and radius as the washer would give a more positive location and no worries about getting them in the same position each side,to adjust,slacken the nut a couple of turns,release the washer and select your revised location and re tighten.A simple suggestion sometimes helps trigger another line of thought sometimes when your trying to solve a snag!!!
Cheers for now.