Could you make something that puts out smoke through some slotted pipe around your shed so they'll think there's a bush fire on the way and move over to the neighbours place?
Most snakes don't like noise so making some as you move around usually has them heading away from you. Are they a dangerous species, or could you learn to coexist with them? :lol: I love snakes but unfortunatly my other half has never let me keep one as a pet. Personally I reckon they've been trying to make friends with you by keeping rats away from your precious grain stockpile, that's why you haven't seen any
The very best way to keep them away from where you are though is to make sure there's nothing in your yard attractive for them to hide in or under, such as old cars, roofing iron, and wood piles.