John Roan has more detailed instructions on his website, www.mountainultralight.com. There he shows coating the strap pads with baby powder to slide them in, and sewing a line of grosgrain down the front of each strap, sewing through the fabric and foam at once. Those stitches not only make useful attachment loops but should help anchor the foam to the strap fabric and make them less likely to pull away. I prefer this to grosgrain edging but that is just me.
I think there are two considerations for the height of the pack: what you’re going to put in the bag for a frame and the seam allowances. My closed-cell foam torso pad, which is from the 1980s, is 23.25” wide. This may be a little wider than most---I think a lot of standard foam pads are 20” wide. I roll this up and use it as the structure of the pack.
The seam allowance is the big thing to take into account. You will probably lose an inch at the bottom (1/2 inch hem rolled twice to encapsulate raw edge) plus height lost from the squaring off of the bottom, and an inch on the top, especially if you’re stiffening the top closure with a plastic placemat strip or ½ inch webbing. I understand you have to roll the top at least three times before buckling to get a true waterproof seal.
With my combination of CCF pad and the seam allowances, I can just barely roll the top three times and I haven’t left a seam allowance on the top edge of the prototype, so I’m at least an inch short.
1. John’s model shows the pack body being 36” x 38,” so one solution may be to make the 38” side the vertical (it’s currently the horizontal on my prototype).
2. Purchase one additional foot of cuben to have some more wiggle room and make an extension collar with it. Unfortunately for my pack no one seems to have the bright leaf green cuben anymore!
3. Cut down width of torso pad.
4. Instead of a drybag loop closure, attach clips to either side of the pack and clip the top down (like BPLer Clint Hewitt’s lime green MYOG pack @ http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=53665). This takes a little less fabric to do.
5. Instead of making a whole pack out of paper bags, just do a quick paper mock up of the body (no straps, webbing, pockets, etc). It will probably take you a half hour or so. Run a strip of duct tape along the top of the bag to simulate the stiffener and attach the buckle(s) to loops of it on either side. Then you can stuff your pad in and try to close it and see if those dimensions work for you.
Best of luck! I’d love to see how you did the curved shoulder straps.