Partial/half frame bags

I’m looking to extend the cargo capacity of my Topstone and am considering a (custom) frame bag. I want to leave room for a water bottle on the seattube.

I’m looking at something like the following…

Is anyone using something similar, and how do you get on with it? I’m particularly interested in how wide it is and whether the rear of the bag flops around at all given it’s not fixed to the seattube. I’m also considering whether or not to use the bottle cage fittings on the downtube to help keep it in place.

TIA

Haven’t used that kind of half frame bag myself but I’d either look at having it slightly undersized, so it’s taut when installed, or some kind of plastic stiffer in the rear panel to stop it flopping to the sides. Or both.

Bolting on using the bottle cage mounts is a good idea, less straps on the downtube to get sheep poop on them!