WelshKiwi – Given the rules you have to follow, I think you are doing the right thing with the filler cap.
If anything, rather than try to blend it in with the body work, perhaps you should make it stand out more?
Sorry these photos are not great, but there were the first ones I could find.
I’d be tempted to add a full metal surround to make it look more like something bolted on.
Something like this…
Or this…
Good luck, Paul.