Hatch bags are fine but only if you must install an access port to start with. Having access ports allow my boat to breath better which keeps my boat dry on the inside year round. This is a good thing.
The idea of cutting a hole in a good hull and installing an access port just so you can gain a very small storage pocket is not a great idea.
If you are determined to go forward with adding one, let me share with you what I have done with some older 16's.
I use the curved based access ports to help maximize contact with the hull.
After cutting the correct size hole and sanding down any roughness left from the cut job, I flip the boat upside down and stage the boat on top of some sawhorses. This make gravity your friend.
I like to reinforce the hull where the hole was cut with two pieces of main batten stock shaped to match the interior of your hull about 2" from this new hole in your boat and perpendicular to the outside of your hulls. Use a small piece of cardboard as a template to get the right curvature and be sure to install these batten "standing on their edge" and reaching just shy of both hulls inside.Now, fiberglass them in place but only on the back side so not to leave any roughness near this opening that can tear the hatch bag. Your hull deck is now stronger than before you cut a hole in it. Now flip the boat right side up.
Now you ready to install the access port base. Be sure not to over tighten the screws which will deform the seal between the access port and the base right at the O ring seal. That will cause a leak into these bags.