I use aluminum drier duct, the rigid kind that's used to go thru walls, not the flimsy accordion duct, with my alcohol stoves and bottle cup. I just wish I could find larger than 4" diameter. 6" would be ideal.

I like that the interlocking seam can be separated so you can re-roll the duct to fit inside a bottle cup or cookset, then it springs back and can be interlocked again, or even spread out to leave one side open to accommodate larger cooksets.