I use a muckynutz long on the back all year round with a bit cut out to allow the chain across without rubbing
And a Crud urban racepack on the rear which attaches to the dropper in the winter over the top of the Muckynutz...
Some (if not all) of the parts here
https://www.jejamescycles.com/product/4497988/cube-cube-battery-cover-holding-washer-rm6an-m6/#search=salesfire&q=Cube%20battery%20cover&&sortBy=&sortDir=
I have dropped mine twice almost didn't notice the first time as I was on some rough ground and just thought it was a bit of debris, the other time was just at the start of my ride and noticed straight away
I can understand how it can fall off without you realising and in hindsight I might...
That's right QECP is at the bottom of the hill but I came from the other direction and turned back at the top of the hill and went back the way I came.