Mine is mounted on the front bumper with a pair of rubber lined circular clips.
This way it can be angled at any position (even flat) if you so wish.
If ever pulled over I could at least have the argument that:
1. The number plate is there; and
2. The wind must have blown it down.
Then quickly recover the phillips screwdriver from the boot and tighten it back into position - just simply too easy
Mind you I may be done for not displaying the plate in a viewable position
- Ah well at least I tried!!!