All the Cashel masks are great. Sure, they come off as easily as all the other brands, and I've tried all the brands! But it is mostly a fit issue in my experience. Put it on snugly, and it pretty much stays put. I like the soft ears, must less irritating to my horses. Long nose version is great for sunburns, but does get a bit hotter.
I ordered horse size which was actually way too big for the intended horse, but it stays on anyway. I have been using it for 3 months with no problem. I am however going to put that one on my other horse with the larger head and order the Arabian size for the other. This is a good fly mask.
Even when my horse was in a paddock by himself he still managed to get it off. When he was out with other horses I stopped bothering with it, because I would have to search for it on the ground shortly afterwards.