what we are doing on the camaro we are building right now.
if you imagine a 20" wide opening just below the bumper that widens out to about 30" at the bottom of the valance with a slightly rounded profile and a tip that follows the outer edge contour and is about 4" wide at the top and vertical on the inside edge with the license plate mounted between the 2 tips
I can post some pics of the opening on monday but here is a rough drawing of the idea
See the third picture down
http://www.killercustoms.ca/images/gallery/69camarov2/main.htm