Some of us use a double bag system. IF you have a backpack that has a separate bottom area for sleeping pad that you can unzip to make ONE big bag, this method serves you well.
 People use heavy duty garbage bags, or makeshift liners but for a few bucks, I would get two of these. Either way you go, get as close to 6 Mil as you can.  
One inside the other, outer bag is your WEAR bag. I fold them them down by gathering them together as close to load as I can, then twisting them 180 degrees, and tucking the excess into the front or back of the load. I repeat this with the outer bag, tuck opposite of the first one. Don't worry about the length, get long one's, you can cut them down if need be.
 I have 4 trips on same two bags, although I had too duct tape a few rips in the outer bag from abrasion by the pack body.
 I agree about external frame pack being unwieldy for tripping, but I've seen in done and IF you have an internal frame pack, you should be fine ;)

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