i still use Dana Gleason's original design of a bag that hangs off a packstrap and crosses over in front of my belly. it's a great concept, if not the nasty trick of sewing.
i demo'd an old wet rib and made patterns, from which it was possible to fabricate a new bag, but this time i installed pockets inside to keep all my goodies off the bottom. the map rides in a pouch inside the outer panel, and that partially protects the contents.
thusly, no need to take off the pack to access map, water bottle, camera, gps, moncular, chapstick, p-bars, etc. just plop your bum on a rock, and you're set handy for a nav break.
all very efficient, and fun to use.
quite the pain to make though. nearly worse than a tent window installation.