How many compartments would you like?
You can add external pockets (1.8L) to the Ortliebs and the Back Rollers have an inside mesh pocket for small stuff as well as a larger compartment against the back (behind the mesh pocket).
I keep my rain jacket either on top of the rack under a strap or on top of everything else in the bag, a jacket really doesn't sink down as fast as a pocket knife (which I put in the mesh bag). Also anything big like sleeping bag, tent, cook set, sweater is easily found. They're only 20L each so it's much easier than a single compartment lightweight backpack.
