What is the impedance looking into a length of coaxial cable that is short-circuited at the far end and a half wave-length long?
Since the joint of 2 waveguides must be created with two 1/4λ lengths, the shorted end will create an open circuit at the joint so that even metal conduction is unnecessary as the current is choked. Therefore an airtight seal gasket can be used.
More useful is the mm waveguide with both shorts 1/4λ apart then at right angles 1/4λ deep on one piece against the connecting flange. It can be rectangular or circular.

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