I found the easiest way was to start with a piece of waveguide, and then stack a series of flat disc-like cylinders on top. Each alternate cylinder was either the inside shape of a recess, or of a ring ridge sticking into the vacuum. Then use the "Unite" tool to join them all together as a single shape. Them put an "air box" around the whole lot. Also, add excitation wave port at the input, and make the surfaces boundaries conductive. That's as far as I got. I messed up on trying to get the simulation to work - so far. You can also use a polyline - a series of lines joined to be the shape of the corrugation profile, and sweep it 360 degrees around an axis.