Embarassingly enough, it was evening when I quoted, and evening when I fitted it. I used glue. What I didn't know was that the plaque copped direct sun all morning, so the glue melted and the plaque fell off.
So. No glue (or at least no structural glue). I wanted it to look tidy; no screws showing BUT with the plaque fitted to the holder, how would I be able to gain access to the screws?
This is what I came up with. Hope it's useful for somebody.