Are the stays the ones that came with the fenders? I've never had stays that touched the edges of the fender as your picture demonstrates. The stays come in a few sizes, matched to the different fender widths sold.
In general, there's no reason for alloy fenders to rattle if installed correctly, so it might be worth going over a checklist during installation.
- Are you using leather washers at all points on the frame at which the fender makes contact?
- Are you mounting the stays directly to the fender and dropout eyelets (no leather washer)? (your picture shows you doing this, this is a no-no)
- Are you pre-optimizing the fender arc prior to securing the stays?
- Are you pre-bending the stays near where they clamp to the dropout eyelets such that they are perfectly perpendicular to the eyelets?
- Are you bending the stays such that they have the same distance from each other at rest as the outer distance between the eyelets?
In your case, I would suggest bending the stays outward at the fender to prevent rattling against the fender edges, then bending the stay ends by the dropouts inward to be perpendicular to the eyelets.