After browsing this forum for over a year and doing some dreaming, I finally got around to making a seat pack. I have some sewing experience, and previously made a frame bag out of the same X-Pac fabric.
I basically made a clone of the sling posted up by JerryW (
http://www.bikepacking.net/forum/index.php/topic,2611.0.html). Several things I learned.
1. Everyone who makes this sort of bag is underpaid.
2. Staples work as a great alternative to pins. With three layers of fabric (fairly heavy), pinning became almost impossible (I bent several and my thumb was in pain after a very short while).
In addition to Jerry's pattern, I added flaps where most straps connect (as his pictures show). I also added another strap that connects to the seat rails to add some stability.
I don't have any overnighters on it yet, but I have used it on a couple day trips to test it out. It shrinks down fairly small and can hold a decently large load as well. It has more sway than I expected, but nothing dangerous. At any rate, thanks to Jerry for a good pattern to base off of!
Edit: sorry for the crappy phone pic, but it at least shows the basic design.
