You have a few options:
1. You can buy or make a mount that fits & secures under the number plate light like this:
Number Plate Mounting Bracket
2. You can drill straight through the boot lid and just bolt it on.
3. You can put some nutcerts in and have a captive thread for the number plate.
I like the idea of using a mount under the number plate light as it doesn't permanently mark the body in case the car gets fitted with different style/size number plates. e.g. if it gets sold interstate to say QLD and the new owner fits US sized plates.
See what other ideas come up.