I have an equinox katahdin backpack (25 ounces/55 liters). I turn it inside-out so the straps and belt are contained. I fill it with other stuff and strap it to my rack, on top of foam cushion. I also carry other gear in front panniers. When I'm ready to backpack I stash my bike, load the backpack and go. I've done this for weekend bike-camping trips.