Before you can even order your New Found Metals ports (NFM from here on) you need to know the thickness of your cabin sides. All NFM bolts are metric therefore I used my calipers in metric mode to determine an accurate bolt size.
You need to measure the exterior off set from the widest point of the old port to the cabin sides. To do this I used my adjustable carpenters combination square as in the picture. Once I had the measurement I locked the set screw.