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All a bit stashed away at the moment so hard to get pics. I didn't do anything too precise. Just cut some flexible relatively thin plastic into a liner kind of shape and put it in the bag so it supports the bottom side. The different strap is just an obvious upgrade I think. It's all about tension with these things and a proper clasp is loads easier to get tight.
Mine also tends to be more up the seatpost than stuck way out the back. The further past the back of saddle it goes, the more likely its going to swing IME.
Here it is in setup mode - excuse the straps that need tucking away...
I sorted my cheap alpkit seatpack with two additions:
It now works fine for bikepacking but I tend to fill it with stuff I will only need in the evening so I'm not undoing/doing it back up all the time.