I've already got a matching pair of hollow ring cavities in my current ship build that are designed to hold rotating gravity rings. They probably won't ever see actual gravity rings, but the space is there and is accessible in the RP interior walkways. (There are no lights along the ring cavities, and they are completely enclosed in the hull, so it's darn spooky in there.) I've even laid it out so that there is room for a 5x5x7 cabin to rotate freely 360 degrees. If it were working, the diameter of the floor of the gravity ring would be 67 blocks, give or take half a dozen. The doors to access the gravity rings are elongated to 3x their normal length, to allow an astronaut time and room to spacewalk to and from the connected hallway on the central part of the ship from the gravity ring. However, you'd still want to hurry, so Dave doesn't get cut in half, caught between the ring and the ship. (I can see astronaut carnage being a major part of the discussion on this subject of moving structures.)