I put a new diesel forced air heater in the port engine room and was working on getting the duct through to the master. I had to locate a hole from the guest stateroom into the air handler section outboard of the closet in the master. My electrician showed me a neat trick to locate holes on opposite sides of bulkheads or for deck penetrations. He brought two “rare earth” magnets with him. Taped one to the closet wall bulkhead to hold it in place temporarily and went with the other magnet to the bedroom bulkhead. Placed it on the wall in the general area and it snapped right to the other magnet. Took the tape off the first and we could move them around to locate the perfect place for the hole. Pretty ingenious method. The bulkhead in that area is 3/4" thick. He said he's used this method for penetrations as thick as 2 inches. Just thought I’d share in case you haven’t heard of this. He claims it works well going thru multiple layers include cored deck to locate thru hull penetrations.