Annoyingly the RRA has no bolt hole on the chainstay bridge and I am NOT drilling it, so advice regarding pipe clamps/similar hacks to get an M5 bolt in there would be welcome.
Of course SJS has a solution.
Though I think I’d rather like to use something that bolts centrally!!! The rest of it is set up as per so would be a shame to hack this last bit.
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.
https://www.sjscycles.co.uk/mudguards/mudguard-bridge-fitting-clip-for-chainstay/