Any thin washer will do, as you tighten things up it will bend to fit the profile of the fender.
I did this (they still cracked after about a year). They were given a tough time on crappy roads and a few tracks.
These new plates should have been specified all along.
Also put one on the chain stay mount. Just drill a lower hole in the plate, another weak point.
Sorry, the fender or the washer cracked? Ultra thin washers work best for me. I should add that these days I'm using plastic fenders exclusively.
@miro_o started
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Any thin washer will do, as you tighten things up it will bend to fit the profile of the fender.