Vetus has changed the numbers since I put mine in. Basically you need the smallest oval one. Because the berth trunk is curved a port that is too long will not sit properly. If you go with the round ports you can use a bigger diameter port. In the end the area of the opening is about the same though.
The port needs to go on the flatter part of the trunk a little forward of the nav light.
Be sure to order the proper sized screen inserts as well. They are sold separately.
For the oval port, use the right size hole saw for the radius at each end, then use a hack saw blade to cut the FG between the two holes. A round port just needs the right size hole saw. Because the Vetus ports are metric, I had to order the correct metric size hole saw online. Installation is pretty easy, but cutting big holes in the side of the boat is a little unnerving.
There is a gallery on this site somewhere showing a step by step installation by someone on their boat.