OK, the picture helps. I think what I did was use a standard license plate screw/nut (nylon). I held the nut with an open end wrench & a small piece of painter's tape (less sticky then normal masking tape). Once the screw/nut was snug the wrench & tape came out easily.
I also removed the entire license plate mounting panel by unbolting the fasteners in the trunk (I happened to have the trunk carpet out at the time). Once the license mounting panel was completely off the rear of the car, it was very easy to attach the license plate (plus a classy frame with a lexan cover) to it using SS washers and nuts secured to bolts that were just the right length. Re-attaching the whole unit from within the trunk was a breeze too.
Ditto on removing the aluminum mount plate first through the trunk and off the rear panel. Then you can really mount the license plate securely and neatly. The plate mount is only held on by 4 10mm nuts under the rear carpet curtain.
One advantage to using the "through the trunk" method is that you can utilize special security fasteners and semi-permanently mount the license plate to the aluminum panel so the license can't easily be stolen from the outside of the car if the trunk is secured.