I've made all sorts of aluminium brackets, but in the end, I've settled on something simple

A piece of PC-ABS with a hole in one end the diameter of the stem, clamped down with the stem cap and a zip tie
Then a couple of bolts to the cage, with a space tube on the rear bolt.

This was my old version https://www.thetrinerd.co.uk/...he-bars-bottle-mount

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http://www.thetrinerd.com