1/2" T45 tubing from ProFormance. That's what I use - see wishbone in photo above ^^^^. Not expensive, easy(ish) to bend, and generally easy to work with.
P.S. I'm about to order a new stash so if you want some, let me know. I usually order 1m lengths to have some wiggle room.
It’s for riding fast, not far but yes I know. There weren’t many slender options on the Columbus catalogue and I couldn’t be bothered shopping around