I like the Ultra Shield stuff myself. It really works for my horse...
I also did try the homemade recipe with the white vinegar and dawn soap. That seems to work.... but it leaves yucky residue and it smells.. I l also noticed, it might make your horse's hair come out .....I'm not sure if that's from it (the flyspray) or from my horse's legs being swollen, or from something else. I just thought I'd say it.
That's just my experience, so use at your own risk. Also, you have to use ALOT and it only lasts a day.
I just found out recently that my horse is allergic to fly bites....a good spray is a must for me now