Ability Scores. Choose one of: (a) Choose any +2; choose any other +1 (b) Choose three different +1.
Size. Large, medium, or small.
Speed. 30 feet, fly equal to your walking speed.
Living Construct. You are a Construct, but your needs resemble that of a living creature more than that of an automaton. You still need to eat, breathe, and sleep. You consume liquid fuel or coal instead of food and water when you would need to eat or drink, but you can still drink magical liquids like potions for their normal effects.
Iron Skin. Your metal skin provides you a base AC of 16 (your Dexterity modifier doesn't affect this number). You can't wear light, medium, or heavy armor, but if you are using a shield, you can apply the shield's bonus as normal.
Living Vehicle. You can serve as a vehicle for up to four characters smaller than you, storing them inside your body. Treat this as if they were riding you as a mount, but they cannot be targeted or damaged while inside you except by another creature inside you or an effect that does not require line of sight.