Originally Posted by
ThermionicScott
Not too silly at all, [MENTION=435871]sjanzeir[/MENTION]. Fender rattle is a serious annoyance. I've used thin pieces of cork sheet secured by electrical tape for the same job.

Velo-Orange has leather washers that serve this purpose:
Leather Washers for Mounting Fenders - Accessories and Hardware - Fenders, Accessories, Hardware - Accessories