a while back i bought an Ortileb seat bag (back before i knew my bikepacking-arse from my elbow) and it was basically a bit crap. always moving around and getting in the way and generally (apart from being really waterproof) no use what-so-ever.
then, whilst recently thinking about ways to make a harness for my dry-bag to mount on the rear (don't know where i got the idea from??? :D ), i had an idea to try bastardising my Ortileb bag to see if i could make something useful from it.
in the end it turned out all i needed to do was change the mounting position and it now works great.
take a look:

the bit that clamped onto the seatpost now clamps onto the frame and the bit that clipped onto the seat now straps onto the seatpost. finally, the clip that closes the bag secures the top to the seat.
perfect!
i realise that this solution will only work for those who (like me) ride a bike with a lot of seatpost sticking out but i thought that some of you may find this solution useful.
i've test ridden it around a little bit but the real test will be this weekend when i'm off for a nice big ride and bivi.
will see how it goes.....