Nick W

For those curious: I ended up with the Pack Strap Pocket from a small gear maker called "Original Free Range". https://originalfreerange.com/collections/bags-and-beanies/products/new-flask-pocket Solves my gripe with the Arva flask holder: backing too wide & stiff Solves my gripe with the Camp flask holder: no adjustable retention via a shock cord

I returned this. The red cord broke the first time I stretched it around a bottle. The panel is pretty wide (wider than my pack straps) and stiff and makes it feel a bit cumbersome on the shoulder strap.