Sounds like a submarine term, which airplanes ain't.
Done some of the things you listed, not others.
If I gave a damn about this time-killer, I'd give you websites of freighter conversions that included sealing some doors and cutting new cargo doors. Find 'em yourself.
Yes - the hatch resembles a submarine hatch ---- as do the baggage compartment hatches and wing escape doors. (Actually, they are a bit more expensive and elaborate than a submarine hatch ... though withstanding less pressure differential pressure. The hatch reinforced frames, and the hatch itself, carry the structural loads of the aircraft as it flexs and moves during flight. (Witness the several jets that have crashed when the baggage hatches fail during flight....)
The cabin IS pressurized continuously to replace the tens of thousands of cubic feet of air that naturally bleed out during flight through the compensation valves, the cabin and baggage compartment, and the atmospheric vents ... ONE more little hole form a mythical bullet doesn't compare to the existing 2 square foot hole ALREADY there in these systems.