just look at the design Altec is using.
It's almost TOO easy, lol.
1. long mounting bracket, with a cut out "track along each side, for the license plate to slide along
2. hinge that connects the license plate holder to the track
3. very simple coil spring , applying contact pressure. plate can only flip up once it gets out toe the end of the track, where the knobs on the bracket are no longer "held down" in the groove.
okay, now going back and reading that, it sounds confusing as hell. if you'd like detailed pics of what it looks like and the diff parts of it, let me know. I'll get them for you. Probably patent infringement though, if anyone cares.