Iron Cross Style Vertical bracket
This bracket is very strong, and will hold up the way you expect it to. The license tag area is the same size as a standard license plate and WILL NOT accept a trim ring. I believe the bracket should be as un-obtrusive as possible, and your Chopper has no room for any extra unnecessary trim anyway. The bracket mounts to the outside of any 3/4" axle.
The light uses a standard 1157 light bulb, available almost anywhere.