look up the term "split block" waveguide. If you want to make a piece of waveguide in two halves, you split it along the centerline. You do not make most of the waveguide in one piece of metal, and then bolt on a flat cover. That is because RF currents are at a MAXIMUM where that cover would attach, and if there is any surface roughness or warping...you will get high leakage. If instead you split the waveguide half way up the side wall, that is a current MINIMUM, and you can be pretty sloppy about the metal pieces actually mating.