Design methods and tools evolved to support the principle of "separation of concerns" in order to manage engineering complexity. Accordingly, most engineering tool suites are vertically integrated but have limited support for integration across disciplinary boundaries. Cyber-Physical Systems (CPS) challenge these established boundaries between disciplines, and thus, the status quo on the tools market. The question is how to create the foundations and technologies for semantically precise model and tool integration that enable reuse of existing commercial and open source tools in domain-specific design flows. In this paper, we describe the lessons learned in the design and implementation of an experimental design automation tool suite, OpenMETA, for complex CPS in the vehicle domain. The conceptual foundation for the integration approach is platform- based design: OpenMETA is architected by introducing two key platforms, the Model Integration Platform and Tool Integration Platform. The Model Integration Platform includes methods and tools for the precise representation of semantic interfaces among modeling domains. The key new components of the Model Integration Platform are Model Integration Languages and the mathematical framework and tool for the compositional specification of their semantics. The Tool Integration Platform is designed for executing highly automated design-space exploration. Key components of the platform are tools for constructing design spaces and model composers for analytics workflows. The paper concludes with describing experience and lessons learned by using OpenMETA in drivetrain design and by adapting OpenMETA to substantially different CPS application domains.