First let me say.... Outstanding job! as well as write-up. I will also be doing this and wondered what the washer spec is? Thanks, Mac...
Thanks. I brought a traced template to a machine shop, made to fit the opening (outside) & another about 1/4" larger than the outer flange for the inside. Sorry, I don't have measurements. Also, I believe the stock is 1/8" aluminum.
I've since drilled a 2nd offset hole to move the Zumo up and to the left. With the offset mounting hole you can rotate the Zumo & position it where you like (vs stuck in the center).
The Machinest is "Bogden" (he's from the Ukraine, and VERY precise with his work). He will remember me & make 2 new washers and mail them to you I'm sure. 805-685-9640
BTW, I just returned from 4,000+ mile round trip to Sturgis & back. The GPS mount worked flawlessly. Easy reach, perfect visibility, no shaking or rattling.