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.