Nigel - three options:
Use a bendy strip of metal from the over-rider mounting bolt, like my rally plaque mounting at the front. Stainless would look good and it can be shaped to be hidden behind the badge if the badge is close up to the over-rider. I'm thinking on the inside maybe.
Fix on the plastic number plate backing plate using penny washers behind the number plate and low profile bolt heads, or allen headed setscrews would probably be better. Use narrow spacers to position the badge slightly behind the plastic backing plate. This may vibrate because the plate isn't that strong.
If your luggage rack has conventional brackets behind the over-riders, have a short badge bar (two/three inches) made up with one end rounded off and the other end threaded, Drill the bracket horizontally - there should already be a small unused hole. Fix the bar with a nyloc nut and Bobs your uncle.
I'd use method three - somebody like Heart of England Morgans could probably do the mini-bar to your spec in stainless.
