I've dealt with a lot of slug destruction, myself, before I got ducks. Seeds didn't even spout out of the soil before destroyed by slugs. THen I got ducks. I usually have about 8 of them, and they free range but usually stay by the house, but I still don't see slugs much 100+ feet from where the ducks are. I'm thinking you could do the duck tunnel, and if you fed them at the other end of the
garden bed, inside the tunnel, and gave them a pail to drink from down there, they would waddle down happily to get the food, and eat slugs as they go back. After a year or two, you probably won't need it, as they'll have destroyed the slug population. (I only see my ducks eating a slug maybe once every three months, and it used to be that there were slugs EVERYWHERE).
How many ducks do you think you'll be getting? I'm thinking 5 would probably do the job very well for you. Even just three might do the trick