Hi- did HArley ever produce a bracket for holding the license plate- see pics from the rear fender/luggage carrier on my 1916 model. Drilling two holes through the carrier is not my preferred solution.
IMG_2846.jpgIMG_2847.jpgIMG_2848.jpg

The Sol rear light removes some options and the light can in principle provide upward lighting for the license plate

Best
NilsR