you could use the pattern for the hip belt of the g4. I like to add 1" to the height and use it upside down. You could also use 1" or 1.5" webbing.
To add it to the pack:
1. Position the belt or webbing against the back piece and pin it to the back piece so the raw edge is against the seam of the back and side pieces. Put the pins at least 1" from the seam of the back and side of the pack
2. Turn the pack inside out. Make sure the raw edge of the webbing or belt is still where you want it and sew it on (you are sewing with the inside back facing you and the inside side piece facing the sewing table. The existing seam should be to the right of the needle and the pack to the left.) Sew 1/2" from the existing seam line. Start 3/4" above the webbing or belt and end at least a 1/2" bellow it. Stitch it 2 or 3 times
3. Do it on the other side.
This is very easy to. Will slightly reduce the interior volume, very slightly. If you want it to look even nicer you can start and end the stitch on the existing seam. Just draw a line from the seam to the new 1/2" seam allowance and sew along it. if the new seam starts 3/4" above the belt start the line from the old seam 1" above that and the same on the bottom.(does that make sense?) Doing it that way will make a slightly nicer transition, but in my opinion isn't that big a deal.
If you're up for a real pain in the butt you can use a seam ripper to open the existing seam and put the belt/webbing into it and close it back up. This is the "best" way, but i would just do it the first way.