Permsteading.com

I'm probably going to try anyway, but has anyone else given it a try? At worst I'll buy a 3m piece for 15€ and it will just collapse under the weight of the cob...

What if instead of building in metal ducts a rectangular profile channel was made into the cob, then "roofed" with floor tile, then covered with more cob?

Like a lot of people I'm trying to be cost-effective/cheap.

The flexible stuff can be detrimental in that it typically is really corrugated, making for a lot of friction in the flue path. If it's a short run it might be okay, but I like your second idea much better. You can make adobe bricks out of cob, or use existing masonry pavers, or whatever. Try to make it as smooth as possible, and it should work out fine. Another option is half barrels, they work great as an arch form for cob, can be configured as flue or to act more like bells, but do have a high volume so can be detrimental if you are going for high mass.