31 mail truck rear fender question

[n2s3]

restoring a 31 mail truck,could not locate any steel rear fenders so went with fiberglass for now. drilled thru the existing holes from inside of the truck, holes came out close to the radius of the fender. cannot tighten down the nut without a radius washer of sorts.I guess I will have to make them unless anyone has seen something like this available online? thanks you,Bryan