I personally just use a tastefully trimmed ziptie for this. They're really unobtrusive, easy to fix (or replace) in the field and hold securely. I have even combined zipties and spacers when the mudguard doesn't sit flush against the CS bridge.
I was ready to naysay this until I read the bit about zipties and spacers, as I probably will need that for the guards to stay secure. Not-so great experience using zipties on my Rockhopper that caused the guards to constantly slip and rub (more than SKS guards do anyway!)
I personally just use a tastefully trimmed ziptie for this. They're really unobtrusive, easy to fix (or replace) in the field and hold securely. I have even combined zipties and spacers when the mudguard doesn't sit flush against the CS bridge.