For the steam stack, we just used your average Home Depot galvanized duct. At both our sites, the kettles have condensate drainage methods that prevent the condensate from draining back into the kettle. We taped the lengthwise and joint seams with metallic seal tape, also available at Home Depot. The duct joints are held together with small, stainless screws.
Our 7 bbl system stack is 11 years old and still great.