I think you can omit the battery in that circuit by substituting two dissimilar metals instead of the copper strips. A single copper wire close to a single galvanised wire will do the trick. The moisture in snail slime will result in a current between the wires sufficient to deter the most persistent hungry snail.
This is similar to the metallic taste you sometimes get if you have amalgam tooth fillings and put aluminium foil in your mouth.