You could try an owl decoy. Keep moving it around from fencepost to fencepost so the critters don't get used to it sitting dumbly in the same spot all summer long. A good garden center should have one.
I've used
"Terror Eyes" from Bird-X in an airplane hangar to keep birds out. I don't know if it'd work on snakes. You could email them and see if they have anything to deter snakes.