Just posting this here so I stop polluting Smithyy's member profile, here are pics of the custom number plate spacer I designed myself.
http://imgur.com/a/q6nchI designed this before Mark had designed his one, personally I prefer mine because it's very stealth and you can't notice it. However marks is a more intuitive design, but unfortunately causes it to be too thick.
The spacer is stainless steel but I sprayed it black anyway, dunno why as it was kind of pointless but I had a can of spray lying around doing nothing so I sprayed it.
The top holes on the spacer are some small square threaded nuts welded on to the back of the stainless steel. And then I bought some new pan head screws from bunnings (was a few dollars for 8 pack) like the ones that fit the number plates already, and had those cut down to the right size so it would end at the very end of the thread on the nuts welded to the back of the spacer.
In the 3rd picture I put my number plate surround back on. Because the bumper curves you will never be able to get it to sit perfectly flat for the whole plate, so that's as best as you can do basically.
Oh and I measured it like Mark did for the gap between the top section and the body, mine is just under 4mm, Marks is 6mm for comparison.