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I bought a 5mm cobalt drill bit, then it was a piece of cake. Just set your drill speed to slow and take it easy to stop it slipping. Had to drill at the brake bridge too. I've used one hole to start with but two would stop the flap rotating, not that it does with a washer and a nylock nut. Domed bolt for tidiness.
@Scilly.Suffolk
I think that's just the angle. These are the 37mm guards over 28mm tyres, which covers better than the 28mm PDWs did. The rear guard sits further from the wheel than the front on my frame, so it doesn't wrap over the tyre as it might on others.
I also covered up the white logo with some black reflective tape.