Spaceships Building Troy [spoilers]

[isf]

In John Ringo's Troy Rising series (non-canon prequel to Schlock Mercenary), they have some rather large spaceships but I'm unsure of how to build them.

The basic hull is a 9 km sphere of nickel-iron with an open interior of 6 km and walls on 1.5 km. As the series progress, crew quarters, engines and the like are added. The main weapon is an huge array of mirrors that focus increasing amounts of sunlight through an aiming device inside Troy (the beam goes to 1.5 petawatts IIRC). The Troy masses about 2.2 trillion tons Based on the mass of the ship, it should be SM +27 with most of the mass as iron? armor.

How do I represent the 6 km sphere of mostly empty space inside; it has some equipment inside, but is mostly used to park ships.

With as SM of +27, even one habitat gives 20 billion cabins according to Eric Smith's Spaceship spreadsheet. I assume that I'd need to subdivide one habitat module into multiple types of smaller modules (habitats, weapons, power plants, etc..)