Use 3M 5200 but set the port in place with the 5200, clean up the residual with mineral spirits and let it sit WITHOUT bolting it in place. The 5200 will makup any difference in the shape of the hull and the port. After the 5200 cures (48 hours), drill and put in plastic screws - sealing them up as you go. With the port base properly mounted, you have a much better chance for the port lid to seal properly.