If you can stand a bit of chem- mix 50-50 flour [any kind] and Sevin.
Store in a small cellar that has a closed extra cap [like oderless spice
jar] taping closed most of the holes; very tight cover. Put a pinhead
size dot about 4" apart [snails, slugs dont go very far!] Its a bait.
Repeat when necessary.

Also works for sowbugs, many stem eating beetles. etc.

My small small amt of mix has lasted 4 years- lots left to use/