Sea to Summit waterproof compression sack on the rack. Actually it's a bit more complicated than that because there's also the sleeping bag, isn't there? So we have a plastic bag inside the compression sack. First, the sleeping bag goes into the plastic bag which is then pushed to the bottom of the stuff sack and then the tent (often wet) is stuffed in there after it and then the whole thing compressed down to a nice size for the rack. We have a piece of rubber stair tread material, ribbed side down, zip tied to the top of the rack to prevent wear between the rack and the stuff sack. Works perfectly. I say "we" because I'm talking about our double down bag and 3 person tent going into that stuff sack and then onto the rack on the back of our tandem. And it's not really a double down bag, but rather a single rectangular down bag that zips to a ground sheet with pockets for our Neo Air pads. The ground sheet and pads go into a separate WP stuff sack which goes in a pannier. We use the same tent and sleeping system for hiking.
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