I would ditch the mounting hardware altogether, drill a couple of holes* about 1.5cm apart acrossways, and use a cable tie round the brake mounting bolt.
If it's a problem of the 'guard having a square profile and the brake bridge being rounded, you can mould a thermoplastic 'guard to shape using hot water (or a hairdryer) and fingers.
* or better maybe, cut a pair of short slits. On thermoplastic guards like SKS you can do this very easily with the heated tip of a penknife blade. Cut from the inside of the 'guard.
There isn't much clearance though; probably under 5mm and I have had problems on v. muddy roads with mud gumming things up badly enough for me to have to stop and clear it.
Put the minimum clearance at the leading edge (ie, the back) of the mudguard. I have mine approx
1mm. Nothing gets through. Of course if you have mudflaps fitted this isn't going to work.