Put a standard port, not curved, in the hole goobered with 5200. Rivet or through-bolt the part of the port that does rest on the hull without bending the port. let it sit. The 5200 will seal the lip and you should use enough 5200 so that it doesn't create a hanging edge to really grab things. It will leave a lip on the edges that can't be flush with the hull but you have solved the problem. Put some through-bolts into the area if you want (but probably don't need to with the 5200 there) but don't torque them down to the extent it tries to squeeze the 5200 out. This will work, but like I said, will leave the edges up.