Maybe a spiky barrier? Like the nail boards beekeepers use to keep skunks from camping out at the hive entrance. I saw someone said rose bushes trained along an access route ended their snake problem. Metal lath is sharp enough, even if you don't use it for a predator apron, it might be cut into strips and attached along the bottom of your fence to keep snakes from crawling through or climbing the fence.
If you take out the snakes, you likely will have rat and vole problems.
If you take out the snakes, you likely will have rat and vole problems.