I have agonised over what to do about the number plate… In the end I decided to copy the current crop of bikes that hang it off a bracket off the swinging arm. I quite like how CCM do it so have sort of copied their design. I have used thin wall steel tube which is always difficult to bend, so to prevent kinking I fill the pipe with sand. First I block one end off, and weld a nut on the other. Then its filled with dry sand until its below the threads in the nut. I then hold the tube against the bench grinder and the vibration compacts the sand, so I fill it again. Finally a bolt is tightened onto the sand which compacts it furter.
Doing this means its far less likely to kink as its bent!