Print this in TPU 100% infill flat on the buildplate .. unscrew the bottom screw that holds the fender and only loosen the one above..

slide the printed mudguard through the rail under the fender far enough that the metal part on the inside of the fender overlaps and then screw back in place..

you can also just presfit it in the rear fender will stay in place perfectly

and presto no more mud problems