Universal modular stented graft assemblies are assembled, on site, and often in a patient's parent artery, from at least two components; a first component and a second component. The first and second components each include a window, or fenestration. The second component couples with the first component by fitting at least partially in the first component to form the universal modular stent graft assembly with an adjustable collateral opening. As the first and second components are assembled, the first and second components are adjusted relative to each other so that the first and second component windows overlap to form a collateral opening whose size is selectable, depending on the overlap, with the desired position and dimensions in the universal modular stent graft assembly.