A "launcher" is used to couple the microwave energy from a magnetron into a rectangular waveguide. It's really nothing more than a short bit of closed-end waveguide with a hole in it (a suitable distance from the end) for the output stub of the magnetron. This one is made to accept a standard microwave oven magnetron. I don't want to give exact dimensions since it's based on a couple of commercial designs, but you'll be able to find datasheets on similar launchers by searching for "magnetron launcher" in Google.
Note that I currently know zip about the behaviour of microwaves and the proper design of waveguide components, flanges etc., so it may be that what I've made is totally useless! (For example, losses at the walls since it's only steel, not copper/brass, and a poor mating flange, since it's made from thin sheet and isn't perfectly flat.)
It's all made from 1.6mm thick steel sheet, cut out by hand and then TIG welded along the seams and ground flat. The waveguide is size WR340.