MikeyMK

I have an alloy grab cage, it's ideal for the job. It grabs any bottle regardless of girth (up to about two litres), though for very thin or wide bottles it needs a little 'adjustment' in the form of reform-bending. It's a cheap and common design. The retaining hook is plastic and can be removed (some may need to be cut off) so the bottle sits flush against the spine of the holder.

I don't use it, though. Some of my jeans catch on it. I use a normal wire-type alloy cage. It's not very flexible but it holds the drinks i buy (smoothies, ales, etc).