I also have a larger version from Alpkit, this has the advantage of being able to set the front strap higher which reduces the flop. But again it's hard to get it tight, even on a fat tubed bike.
I sold the large version of the Alpkit bag and bought the medium (Oddly there isn't a small!) Despite the differences in dimensions seemingly minimal, there's quite a bit of difference when you see them side by side. Used it over the weekend on two Audax, it's just big enough for my camera (Compact, but not the smallest) and a small power pack to keep the Garmin charged. It isn't perfect, slightly brushes my thigh when out of the saddle and can get a little knocked askew when I have a foot down at a junction, but not by much. Of course it doesn't have the same easy on/off of a bar bag and isn't really as accessible on the move. On the whole I like it and will continue to use it for day rides, for touring I'll probably stick the bar bag on and look for an easy way of adding and removing the fitting.
(https://farm1.staticflickr.com/873/40284036415_22e93769b8_n.jpg) (https://flic.kr/p/24nLdxZ)Alpkit bag (https://flic.kr/p/24nLdxZ) by Paul (https://www.flickr.com/photos/phbike/), on Flickr
other colours are available
https://www.alpkit.com/products/fuel-pod-medium
Just seen it's gone up by £7 in the week since I bought it ???