My horse lets his neighbors take his fly mask off. It either gets ripped off in one piece, or is torn apart slowly piece by piece. He has been wearing the Kensington Catch Mask for a month now, and it has stayed on. I wanted to let you know in case you have the same problem. Also, it’s on sale:
http://www.horse.com/item/kensington-catchmask/E001050/
The only drawback is that I bought this for an average sized quarter horse and I had to get the large.