Ok if this were my horse, and I was paying board, I would flipping well be shouting my head off at the BO for not properly maintaining her facilities so my horse could be comfortable. If she won't let you put a sheet on her, or a fly mask, and her stall floors are mud and muck which is going to end up causing your horse to get thrush, I would just move my horse.
Spent a whole hour today laying in a pasture, waiting for a sparkling vampire to show up. Alas, I woke up and looked over, only to find a mound of horse crap. Sigh.