OK. I guess a lot of folks have never used an external frame pack. When using an internal frame, you put your sleeping bag in a light (say sil-nyl) bag and then put that bag in the bottom of your pack. Your pack acts as the outer bag. With an external frame pack, you use a stuff sack and tie this on the bottom, under the packbag. Bulky stuff goes in here - be it sleeping bag (stuffed in its own bag), tent, pots and pans, etc. The external bag takes a beating, so will not stay waterproof long. Similarly, an internal frame pack takes a beating and will not remain waterproof - hence - they are usually not made of waterproof material. Since the bag I want for my external frame pack will get ripped and poked, I am looking for something tough but light and mainly cheap -ideally cheap enough to be disposable - simply throw it out after each backpack season. Tyvak comes to mind, but it is hard to buy small quantities. The large size oxford cloth bags sold at REI weigh about 4 ounces. But it is not cheap - yes cheaper than sil-nyoln. I am just trying to think of a better solution.