I've never done that kind of licence plate holder in that position. But if I were to do one exactly like yours , I would consider soldering or brazing a piece of flexible metal tube from the top of the bracket at the light, attach it to the radius behind the plate, add a offset solid tab to be held in place by the mounting bolt w/lock washer, continuing the run either on the side or underneath the swing arm to a hard bend just past the swing arm bolt and into the V in the frame . This is all for housing the wires, I would probably make a small bracket and attach it to the front chain guard bolt on the swing arm instead of a zip tie, paint it black and run wire through it. But that's me shooting from the hip. You have problem solving skill , why give up on your design so quickly? No pressure here, you have time, see what you can come up with.