Have the same problem with backpacks so gave them up for the summer. Just got a Timbuk2 messenger bag (don't know what it's specific type is called but it doubles as a pannier and hooks on my back rack). When I'm on the bike it simply hooks on the rack, off the bike, the hooks are hidden with velcro straps and it's a great messenger bag. Bought it online from REI. So goodbye sweaty back.