I haven't made one but it would be very easy to do. I would use 1/8" aluminum plate (since I have lots on hand & you can polish it if you are so inclined).
Trace the shape of the license plate on the aluminum and then draw on an extra section for a mounting tab to the bike. Cut out shape, drill mounting hole in tab, drill holes to mount license plate, bend tab at a 90 degree angle to the plate. You can mount it to the top or bottom shock mount.
I like my plate mounts to make a full back plate for the license plate as I think it looks a little nicer so that's why I use aluminum plate. If you don't care about that you could just use a strip of band iron bent & drilled to mount on the shock mount.
You may want another tab above the plate to mount a plate light.
Make sense?