Hi Anna and thanks for your question about copper and zinc repelling slugs. One of the problems with slugs is that they don't always enter a raised bed from the top, so the copper may not come in contact with them. Then the copper must be wide enough to connect with the whole slug and some of the copper bands sold for this are not. Zinc does not provide the same chemical shock so would not work.

I'll attach two articles with ideas in case you don't want to go with the expense of copper (which might not work if the slugs find a way past it).

This one has more information than you might ever want to know about slugs, but is local and effective as it is research based:https://solvepestproblems.oregonstate.edu/plant-problems/slugs-snails

This one has much of the same information from Minnesota and is in a more abbreviated form: https://extension.umn.edu/yard-and-garden-insects/slugs#slug-barriers-298063

I use beer traps, non-metaldehyde slug bait and "squash when you see them" techniques. I'm just not excited about going outside at night to hunt them and pick them off!
Good luck,