One promising approach is the No-Mud. By installing on the struts of the rear triangle with it springs and remains always the same distance to the tire. What sounds great in theory fails in practice in a mountain unfortunately on a small amount of plastic is missing the top result.
I drive a 26-inch mountain bike with 2.3-inch tires, and for the apron is in the back of (the protective plate is narrower towards the end) is too narrow and too short so I never got clean after driving through the mud at home. By adhering a small home-made spoiler to the left, although much improved, but it was not optimal, so the fender was again removed and replaced with a Zefal Deflector. Which, although designed for 29 inch, but fits perfectly on a 26 inch wheel (and interestingly, 5cm too short for a 29 inch bike).