That's helpful, but I want to build a rectangular shape pyramid horn antenna (waveguide + flared horn), as it's easier to build.
The microwave input will be straight from the magnetron, not N connector.
Have a look at "ingenuity unlimited" run by Bob Iannini, there is a design for a magnetron horn described in his book "more electronic gadgets for the evil genius" The device described has multiple ways of killing or seriously injuring you and is not suitable for anyone who does not know how to safely work on such things. Uncontained and unsheilded microwaves at the energy levels obtainable from a magnetron can damage eyes, boil and explode testicles, cause severe cellular damage that is irreversible!
Oh plus the voltage used can exterminate you instantly should you come into contact with it.