The lip is part of the stock XTR derailleur. It does give the square brass nut some nice purchase so it doesn't rotate when you tighten the cable down. 6mm is the magic number but the cable needs to stay lined up with the stock line pull. I originally started this mod with a stack of washers at first and quickly found when the cable inevitably rotated away from the stock pull direction the pull ratio was off and the shifting suffered.

 There appears to be a lip on the derailleur that limits how wide it can be in this direction. How far is that from center? The only other critical dimension is 6mm height, right?

 I could not find a square washer at my local hardware store. Where did you get yours?

 I'm all over this!