There are so many different ways to attach an opening (removable hatch, deck, structure, etc.) to the hull. All depends on what you want to hold down, what it is, how tight (waterproof?), shape, etc. Without showing what you want to do it is impossible o recommend any particular method. I have boats that use magnets, friction fit, gravity, screws, latches springs, etc. So many different ways. The main idea is that it is concealed!

Added: My 42 inch Schnellboot boat has multiple ways to get inside. The manufacture's plans showed one massive removable deck section with ugly plastic hold down tabs and screws. Check the photo and you will see the various section openings. (This also makes the hull stronger. Downside is it is a tight access, but several months I managed to do a complete re-build. Battery access is not too bad but I usually do that at home. Power switches are under the pilot house which has a snug (friction) fit.

I just watch three YouTube videos about the Narvik model. If you are looking for a way to access your motors, batteries and such below deck I am not the guy to be giving you advice.

None of my models are this big!

3 of my boats rely on friction between the cabin and the hull. The fourth relies on magnets.

Another approach if there is some feature that rotates is to make it into a latch so that when it's rotated into the correct orientation, it locks the deck hatch down. I've seen this done with secondary turrets on warships.

I should add that a lot depends on where you run. If you run where careless folks might ram you, then something affirmative like a latch makes a lot more sense than friction. A good impact, knocking the cabin off, can ruin your week. Where I run, collisions are rare and mild, so I don't worry so much about it.

@merchant67 Second landplat’s suggestion of rare earth magnets. I use them all the time. I mount one under the removeable deck section & another magnet is attached to a deck framing member. Be sure to get the polarity of the second magnet correct. Two RE magnets create a strong clamping force.

No this is for the Deans Marine model Narvik which I am currently working on!!

@merchant67

Is this question in reference to the tugboat you are working on currently?

Curtain hooks and eyes, attached inside the hull stretching across the aperture, then hooked onto a curtain eye attatched to the removable hatch
located with the aid of a Button Hook or such like.. Regards Muddy.

… rare earth magnets; small and powerful…

Drill a hole and pop them in…