A stick on number plate would not be legal because it would not carry the name of the maker, and this is something over which attempts are being made at strict control. I'm not in favour of protrusions from the backs of vehicles of any kind when they are not being used because I have had vehicles damaged by them. But you raise an interesting point because the current model Corsa can be fitted in production with an accessory slide-out bike carrier, and provision must be made for the number plate. So maybe your problem is self-inflicted by not using the manufacturer bike rack.