When we had a problem with slugs in her rosebushes in the Seattle area, the local horticulturist recommended a copper barrier fence. The barrier has to stick into the ground an inch or two, and be tall enough that the slug has to touch it (crawl over it) to get past.

The theory was that the copper breaks down the slug slime and the slugs scratch their bellies. They don't like that and won't try to get past.

At the time, one could get copper "flashing" as used in roofing fairly cheaply, but with the current salvage value of copper I'm not sure it's affordable now. Real copper woven screening might be usable. Much of what shows up in the hardware shops here is actually brass/bronze and I can't say whether that meets the spec for this use.

Galvanized steel was said to work, but somewhat less effectively; and either Cu or Zn will quit working if it gets covered with enough dirt and/or corrosion.

Note that I didn't try this except on one very small plot. It seemed to help, but I can't say that it's fully reliable.

John