Tomorrow's big science systems are in development today. Success depends on global collaboration in which multiple international parties produce unique assemblies. Inter-organizational barriers, interests and technical conflicts often complicate the design and realization process. It is especially difficult to manage a system's interfaces over such boundaries. We propose a model-based approach to support integrators in such situations. Combining a system architecture model, Integration Readiness Levels (IRLs) and a network of actors, we can derive a newly introduced Collaborative Interface Risk Index (CIRI) that highlights risky interfaces, the organizational patterns that are required for successful integration and a progress indicator that tracks how many interfaces have been defined sufficiently. We demonstrate the method for one of ITER's diagnostic subsystems, which is being developed by six distributed organizations. The result is a Multi-Domain Matrix (MDM) that gives a complete overview on the complexity and risks of the collaborative engineering project.