As Glenn said, ignore pack waterproofing.

Consider a pack from Gossamer Gear or ULA:

http://www.gossamergear.com/
http://www.ula-equipment.com/

I have the GG Gorilla, but the Ohm looks good too. I could do a thru-hike with either of these. They weigh around 1.5 lbs each with all common accessories like hip belt with pockets.