another, often overlooked, place to find a great SUL pack is just to buy a closeout Jansport bookbag backpack or similar.
I found a 2000 cu in book bag at Old Navy last year on clearance for $5. It weighed in at 21 oz, but was made from a very durable packcloth and was a panel loader to boot!
I haven't got around to it, but I plan on taking the front pocket off with a seam ripper and replacing it with a web pocket with elastic on the top. I also am considering taking the side pockets off and replacing them with webbing as well, making them large enough for 1 L water bottles. That should drop the pack weight down below 15 oz and increase the capacity to 2500 cu in or so.
The suspension is designed to carry 40 lbs worth of books across campus, so I've used it for my 7 lb baseweight kit with NO problems.
The point is ... don't overlook what you may be able to simply modify instead of make from scratch.