OK, bit late but here are the photos of a 13L alpkit airlok xtra used as a seatpack. The trick is to pack it really tight so it keeps its shape – also, way the first strap goes through the saddle rails gives it extra stability.
here is a pick of the bike on its side, you can see its not flopping (weight of the packed bag is over 6lbs so its fairly heavy). You can see it isn’t too wide so doesn’t get caught on your thighs.
Finally, here is a pic of the first strap going though the seat rails and attachment point on the bag (The second strap only uses the bottom attachment point and then through the seat rails. If you need more stability, you can add a 3rd short strap from the top unused attachment point and again go through the seat rails.
Great value for £10.