Think I've found a clean solution to a an easily removable, hole free number plate mount- magnets.
http://www.mk5golfgti.co.uk/forum/index.php?topic=12447.0
https://www.google.co.uk/amp/www.instructables.com/id/No-Holes-License-Plate-Mount/?amp_page=true
On the Z3 it will probably need the crash bar separating from the bumper skin to place the magnets on the rear of the plastic part of the bumper (doubt the magnet will be strong enough through the crash bar which is thick aluminium).
What do you think @Tim beard
http://www.mk5golfgti.co.uk/forum/index.php?topic=12447.0
https://www.google.co.uk/amp/www.instructables.com/id/No-Holes-License-Plate-Mount/?amp_page=true
On the Z3 it will probably need the crash bar separating from the bumper skin to place the magnets on the rear of the plastic part of the bumper (doubt the magnet will be strong enough through the crash bar which is thick aluminium).
What do you think @Tim beard