Stupid question; where did you pick up the plate backing?
Believe it or not those are a little harder to find than you would think (at least for a reasonable price). The price some places want for a backing plate is utterly ridiculous. The one in the pictures is plastic (I think Custom Dynamics has them for a couple of bucks), but I didn't like it because it was a bit flimsy and had a little curvature to it so I found the backing plate from a chrome license plate frame I had on another bike I crashed and totaled about 3 years ago. Kuryakyn sells it as chrome frame that comes with a nice chromed metal backing plate for about $17 (Kuryakyn P/N 9166).
Better living through chemistry.