Personally I dont care for my plate hangin out there for everyone to see. I have a friend who has had his plate stolen twice. I have mine attatched to the back of the battery box with a nylon flat license plate screw. I put tape on the inside so it wont hurt the battery.
As far as I know this is legal in most states. Mi law says "visible from 20 feet behind vehicle" although law states it must be lit up. I have never had the police even mention it. Other states law say 'visible from 10 feet' but this should be ok.