For a couple of years, on my old Trance, I used a Crud rear guard:
Took off the ‘arm’ which usually mounts to the seatpost . In its place I mounted an old seatpost clamp, one with a single bolt right through it, fitted this in place of the crud arm and used just the ‘ front’ jaws of the clamp to fit the guard to the saddle. Worked a treat, never came off. Cut a piece of old inner tube, zip tied one end beneath the seAtpost clamp an fixed the other end to the front if the crud guard, this pulled taught with the dropper raised and just folded out of the way, when dropped, caught mud and water and kept 90% of crap off the dropper.
Looked a bit crap, but I didn’t care, staying relatively dry on wet days.
Using the same guard with the arm put back on , on a hardtail now. Need to sort something for the new full suss.