Here's what I did for a shoulder bag int he woods:
I started with an old army buttpack. I managed to shoehorn in an MSR Miniworks water filter, pack towel, small first aid kit, bug dope, Gerber ProScout multitool, waterproof match case w/storm matches, a Bic in a small ziplock, 2-50 ft lengths of braided paracord, a Mosin Nagant sling (more on that later) and 50 rounds of .22 magnum. To the outside, on the ALICE loops, I put a 1 qt canteen with cup and stove attachment, and on the other side, a utility pouch with food (3 cans of SPAM, 3 vacuum-sealed pouches of minute rice -- each pouch makes enough to fill 3/4 of the canteen cup when cooked, a dozen bullion cubes and a spoon. When all is said and done it's about as many calories as 3 MREs in the size of one.)
Then I thought, "Dang, it'd be nice to have more of a knife than just that multitool's dinky blade." I felt my RAT-7 staring at me, I tried attaching it by looping the MOLLE strap around the top straps of the buttpack, between where they are sewn at the back of the pack and the horizontal keepers that keep them on the top of the top flap -- perfect fit! When worn as a shoulder bag, it's very easy to reach and take out, when worn around back, it takes a little stretching, but I can get it fairly easily.
Since I hate carrying buttpacks as buttpacks, I attached a Mosin Nagant sling to the D-rings at the rear to carry it over the shoulder. I threw a second Mosin sling in the bag so that I can attach it later. I found if I put it on right, it rides like a backpack with the straps straight on the front, and crossed in back, stabilizing the load.
To the bottom I strapped the bivy sack from the MSS.
I've since replaced the Miniworks water filter with Katadyn Micropur tablets, and replaced the bulk with a tarp.
I offset the weight somewhat by running another sling through the frog on my kukri's scabbard, and wear it slung from the other side.