I can't seem to find how to increase the scrap multiplier of scrap gained from scrappers or barrels. How can you do this?

That's not gathering. That's loot drops. You'll need a loot/drop mod for that. It's a completely different system. As to increasing gains from recyclers, I don't know of a mod that does that off hand, but it might be there. Just search for loot and you will find several loot mods that can multiply drops from containers.