Not true Martin!
The rear mudguard will keep the number plate clean. I should have taken a picture at the time but never thought.
My K12S and a mate on his K13S had ridden exactly the same filthy few miles when we stopped for fuel and when we came out of the kiosk, with the bikes parked side by side, the difference was alarming.
Neither of us knew about the rear mudguard thingy; I didn't know mine had one and he didn't know his didn't. We were well confused and as I had been following, I would have expected my bike to be muckier in the turbulent air but not so.
The back of my bike was virtually spotless and his was so bad you couldn't read his plate.
It was only on closer inspection that we spotted the small plastic infill panel and that was that.
Obviously, doing your mileage it ain't going to be perfect, but it might keep you alive in the filthy dark and for only £20 or so and easy to fit in your case as you don't have the Sport Case frame to contend with.