GillyScott,
I use the Helly Hansen duffel line and they are bombproof and huge. I'll be taking the HH 90L to BVI in March with a Marmot H20 resistant backpack (Clearwater 35L). Helly Hansen also makes 30L and 50L sizes all with /backpack shoulder straps. One line is water resistance(what I use), the other waterproof but you will pay for it.
http://shop.hellyhansen.com/us/item/hh-duffel-bag-90l-67004/
http://www.amazon.com/Helly-Hansen-Duffel-Bag-Navy/dp/B00413PF20?tag=foodscatal-20

~Eric