At the end of the build I went online to find a suitable scrambler cafe like rear light unit.
New Unit installed
I came upon a standard item with a Rubber base and then fashioned a bracket from it underneath to secure the numberplate.
Not long after I got a numberplate back but on the whole they were all too heavy and repeatedly broke, wobbled around were unsatisfactory.
Back to basics and I secured a stronger solution and fitted it. This one bolts straight to the Mudgaurd and gets rid of the strap and also I think looks old school.
Just my opinion of course but much more satisfactory.