The instructions for fitting Portland metal mudguards says drilling a hole for direct mounting weakens it. They even sell a crimp on brake bridge bracket for direct mounting.
My suggestions:
use a brake bridge bracket or
direct mount:
make sure the surfaces around the hole are smooth to reduce stress risers,
use large washers, shaped to fit the mudguard curve, to reduce tension on mudguard
use rubber and/or leather washers to reduce vibration, although I would have thought rubber would be better.