You might try Gossamer Gear's website; at one time they offered the pattern for the original "G" model pack (G4?). I've never tried making one (when I do something myself, it ends up looking like I did it myself.)
Check the Make Your Own Gear forum here; I'm sure you'll get some good ideas.
For less than $120, you can buy a frameless pack from Granite Gear (the Virga) that I have used and really like; I just can't keep my load reliably below 20 pounds, which was the limit of comfort for me with that pack. There are also some other folks (Gossamer Gear and Six Moon Designs) that sell online, and seem to get good reviews.
At one time, TarpTent gave away the pattern for its original tent; I don't know if they still do. (Franco - do you know?)