If you tap on the hull with something like the handle of a screw driver you should be able to determine the center of the bulkhead, like finding a stud in the wall.That is providing it's still attached.

I doubt your going to want to cut a hole in front of the beam. I've dealt with this ( not on a Nacra) by going through the port behind the beam. Take a dremel tool with a spiral cut bit and cut just the corner of the bulkhead out. Take as little as possible. You should be able to get to it. After your done you can glass the corner back in.