Why wouldn't they fit through the door? They look like they were sized with the doorway in mind - narrow and tall. Isn't that also the reason to have 5 of them, because 1 or 2 bigger ones couldn't pass through the door? Too bad we don't have detailed drawings of their construction - there are some important questions they might answer. Would they float, how did the dump valves work, etc...
The odd-colored panel is interesting in its own right. If it were an opening, then the underlying structure (ribs, stringers, etc) would also need to be removed, which either compromises strength, or adds weight of reinforcements.