A friend of mine always rides with one on her horse as he has uveitis and she was apparently told to do this by the vet. It's ok as long as he doesn't get too splattered with mud out riding so that he can't see out!
You can get the proper ones on ebay, they just velcro onto your bridle. I used one on an ex racer who hated flies, he would violently throw his head around and walk with his head litterly under his chest to get away from them. It worked a treat, a happy horse and i wasnt getting yanked out of the saddle with his antics lol.