I went with the same motorcycle rear light that you're having trouble fitting Paul.
I just bored a hole for the rear extension that carries the cables (you can see it in the middle of the back end here) -
I also flattened the mounting bracket as it's angled for fitment to a curved motorcycle rear mudguard, then used the mounting bolts supplied to bolt the numberplate to it and through the shell to secure the whole caboodle to the car. Quite simple really -
