Gain Horn for WR975 rectangular flange to a circle

I need to find a fabricator somewhere around the Washington DC area that can build a gain horn transitioning from a WR975 to a circle. The circle will be going into the oven. Would it be better to have the circle bigger than the WR975 or make it fit inside? I would like to lose as little power as possible. If anyone has a blueprint or an example of what it should look like that would help tremendously.