I got the feed sacks stitched up. The old Singer 99 and nylon upholstery thread did fine. Now I've been playing with different ways to mount and load the pack.
The feed sacks have about a 13" diameter and are about 25" long. When the overlap on them is adjusted to make the shortest sack you get about 2850 cu in of storage. It can be adjusted it to 40" tall and still have 4 inches of overlap and plenty of enough room to fold the top shut and that gives you about 4500 cu in. Those are conservative estimates based on what I figure you'll actually get out of it. (I took an inch off the diameter in my calculations and rounded the numbers to look nice). I give a more accurate weight after I finish and cut all the extra strapping off.
Adjusting the sack so it fits pretty much centered vertically on the frame seems pretty comfortable with a 27lb load. That's as much as I've stuffed into it so far.
As you said, it's not as convenient. It's a top loading pack with no pockets. I think I can get used to that, and make the best of it by using ditty bags organized with stuff. But I do like conveniences, so I've been thinking of ways to attach some pockets.
Because of the "telescoping" sack design, and the straps used to compress the load and attach it to the frame, sewing pockets on it is not a good option. So I'm thinking of a sort of "Tool Belt" type set of pockets that are removable. It could hold water bottles, compass & maps, gps, phone, snacks, whatever, and be made with the same feed sack material. It could be attached with straps after everything was packed and cinched down.
I'll get a chance to give it a decent test it in a couple weeks. It's such a hillbilly looking rig, I can't wait to try it out