I didn't have much success making backpacks either. Ended up buying a GPV4 (gossamer gear, now) and a Golite in the end. When the UL bug first bit me, I did have success modifying book bags....which are really day packs....bought from Walmart or Target originally for my kid's school needs. When they replaced their book pack, I'd recycle the old ones by lightening the heck out of the things. Cut away anything that didn't seem necessary, added net pockets, whatever. Those bags actually made some useful 1-3 nighter packs that I still use once in a while....and they are really tough, and respectably light weight.
Price is right too. <img src="/forums/images/graemlins/cool.gif" alt="" />
paul, texas KD5IVP