Choose ripstop polyester, nylon, or DCF. Thanks to a special impregnation, these materials are breathable, waterproof, lightweight, and do not fade in the sun. What I really like about them is that even if you get caught on something, and your bag rips, that hole will not spread further and will not increase in size. Backpack hardware should be strong (preferably marked "YKK") and withstand low temperatures without losing strength.