I still have the stainless bracket (original) with the number plate mounted directly on it and the light modified as shown , I think this is better than some of the brackets I have seen for mounting under the rear light as they look pretty flimsy.
Also , if you remove the original mount you are left with the mounting studs protruding from the standard silencer , not pretty
If you have a termi silencer fitted you are left with open threaded inserts , or I suppose you could put bolts in them , either way is a dirt / corrosion trap.