Just built a cardboard template for my first attempt at a frame bag. Hope to do a one-piece design (or something close to it) as shown by Greg Climbs in his bike packing 2.0 photos on Flickr.
Bought 1000 Cordura for the outside and silnylon for the liner. Will use plastic stiffeners for bolting it to the water bottle cage bosses on the seat tube and down tube. Plan to use Velcro on the top and a single horizontal zipper on the right side.
I am thinking of sewing a partial vertical divider inside so I can slide a 3-liter bladder in the back of the pack, just forward of the seat tube, and be able to pull the drinking hose out the top.
I want to keep it simple and sturdy. Did not know about the X-PAC fabrics when I bought the Cordura and silnylon. It will be interesting to learn about the advantages of X-PAC.
Please post up comments and recommendations, if you have them! Thanks, Joe