Found this from my usual vendor (it hadn’t occured to me to look at them before) on Aliexpress. Those looking for CF tube for Delta arms https://www.aliexpress.com/item/6mmx4mm-length-330mm-carbon-fiber-pipe-carbon-rod-hollow-tubes-carbon-fiber-rod-airplane-model-kite/32764397447.html?spm=2114.13010208.99999999.262.OO7RY1 - I just picked up 6 330mm lengths for £4.73, shipped. I’m pretty damn sure that they’ll cut it to the right length if you contact them too.
The 330mm will give me a good middle ground for the short and long arms for the #AX-02 Delta.