A method of forming a monolithic aircraft part includes additively forming a first cavity, defined by a first wall of the part, additively forming a second cavity, defined by a second wall of the part, so that the first cavity and the second cavity are not in communication with each other and share a common wall portion so that the second cavity is partially contained within the first cavity. Additively forming the first cavity includes forming a first aperture, extending through the first wall, forming a second aperture, extending through the first wall, and forming a first inner surface of the first wall, defining a first shape, wherein a first object, permanently having the first shape, would not be able to be mechanically extracted from the first cavity via the first aperture or the second aperture.