When my son was in Boy Scouts, the troop used to transport gear to the starting point for trips in an open truck, including in wet weather. I bought jumbo sized Ziplocs that fit his frame pack when I adjusted the frame to the smallest setting, and could be folded up and put in the pack during the trek. These were effective at keeping his pack and sleeping bag dry. I think they were called the 3XL size. If I was using one on a checked frame pack, I'd run a couple loops of strapping tape around it.