Sounds to me like you need a zero one pack from Truenorth outdoor.
These are designed and made by a mate of mine called Jed Yarnold. Well known in the canadian canoeing fraternity, he used to design equipment whilst serving in HM forces.
Now retired from his military life, he designs equipment for his expeditions. The zero one pack is pretty much bomb proof. Simple in design strong and... waterproof. The prototype was even used as a sea anchor attached to a line and thrown overboard whilst full of kit.
The best thing is that they are designed to stow well (in a bow, under a thwart on a landy roofrack etc.), have loads of attachment points. They can be carried like a rucksack and then the straps all pull tight or stow away so that they are not trailing everywhere. You can probably tell that I am a bit of a fan! Pm me if you want further info. I may be able to arrange mates rates.