I haven't waded through all the responses, but if a surplus army bag hasn't already been mentioned let me know and I will shoot a couple of pics and post for you. I have a 'newer' infantry bag and it is just about perfect. More configuration options than I could possibly list. Straps, pouches, etc. everywhere.
Mine has 3 days of food, water, shelter, tools, and ammo with room to spare in a bag that is still about the same size as a standard backpack.
Let me know.