I ordered this this morning:
http://www.sierratradingpost.com/columbia-sportswear-river-runner-xl-dry-bag-backpack~p~4418m/?utm_source=GooglePLAs&utm_medium=PaidShopping&utm_term=Columbia_Sportswear_River_Runner_Xl_Dry_Bag_Backpack&utm_campaign=PCGOOGLEPLA2&codes-processed=true
If it looks tough enough and secures to my frame OK, I think it's going to be awesome! It looks like I can roll down the top to custom fit for bigger or smaller loads. It weighs 1lb, 8oz - not too bad! My pack frame weighs around 5lbs but carries a heavy load when needed. Some of the old Jansport and Kelty aluminum pack frames weigh well under 2lbs and some closer to 1. I'm going to try to pick one up to lash this pack on to for scouting trips, off-season hiking, and maybe even hunting depending on how they handle a load. I've seen some threads online where people have changed out the waist belt/straps for more comfort and to save weight. I'm a minimalist, so I really dig this stuff!
The Columbia dry bag was a bit too small, so back to the drawing board to find the right bag(s). I used a large nylon stuff sack as a test and put everything I wanted in there and lashed it to the frame with my daypack on the outside. It works pretty good! I packed my sleep mat (closed cell foam) first - loosely rolled around the walls of the inside of the pack - and put everything else down into the middle. The structure this created rides nice and snug on the frame. Pretty basic and low-tech, but flexible and effective. I'll post pics next time I have it rigged up!