As a bonus question, has anyone mudguard mounted a Secula rear light, and is there enough space with 28mm tyres for it to fit properly?
Yes, not on an Audax, on my old 700C tourer with bigger frame clearances than an Audax, but fitted with 28 mm tyres and narrow mudguards adjusted to be fairly close-fitting.
There needs to be enough space under the rear of the mudguard to fit the nut that fixes the Secula, if there isn't, it should be possible to move the rear of the mudguard further out on the stays, perhaps at the expense of looking less tidy.
Secula is smaller and lighter than my previous mudguard fitting lamps, so less inclined to waggle the mudguard.