I got one of those zefal gizmo mounts so I could put an extra water bottle on my seat post.
The supplied fixing bolts were too long and dug into my seat post .Plus I’m not sure that I wanted to leave a cage on there permanently then spied an old camera bag which a small bottle would fit in. I fixed it to my saddle with a toe clip strap.
It does the job but I got to thinking there’s something better out there.
I think the fabric mounts would be perfect but everyone reckons the bottles fall off.

Fabric mounts seem to work better with the smaller bottle. The larger one seems to have too much weight above the mounts and twists off.

The larger does seem to stay on better if you have the bottle mounts on the seattube, the smaller bottle seems to stay fine on the downtube. Even on a rigid fatbike