I had our packs rained on last night and they were completely dry. Simple sil-nylon pack covers. Our packs are 60 liter but the elasticized covers would probably stretch much farther. The covers wrap around the bottom and a little around the shoulder straps. We just lean them against a tree or boulder. If it's raining lightly while hiking, they go on the packs, when pouring they get covered by poncho/tarps that haven't yet let us down. This system has worked for quite a few years. Our current ones are from Sea to Summit.