use g4 pattern for shoulder straps (and belt if wanted) and back panel then instead of all the other stuff just use 1 piece for the sides and front. Use the square bottom like on stuff sacks (instructions on thru-hiker.com) and add a drawcord to the top.
For an average to small pack, perfect for the ultralighter, i'd go with 10-12" back piece with 24-28" front/side piece. You could use heavier 4oz oxford for the back and 1.9 or 1.1 for the front. You may also want to add a strip of heavier stuff to the bottom of the front piece so once all sewn up into a square bottom you have heavier fabric on the bottom all the way around.
However, unless you have a lot of high loft gear you will probably want some type of compression system, if only an over strap and some shock cord around the body.