10m of silicate ceramic; 30cm of iron. A hull, in that order from exterior to interior, is all you need to get radiation shielding comparable to what you get on Earth's surface. If you build a torus or cylinder more than ~1km in diameter, you can spin it for Earth-normal gravity, something you can't get on a planet. You could even make parts of the hull clear; just use 10m of fused quartz, 1m of either water or poly-carbonate, and 30cm of aluminum oxide instead. All the bulk materials could conceivably be mined on the Moon and launched into orbit electrically with mass drivers. You could power it with large solar arrays. If you use multiple levels inside, you get rather huge surface areas to populate. Just beware of colony drops.