The Slice-based Federation Architecture (SFA) is the de facto standard framework for managing testbeds, federations of testbeds, and users access to these federations. It is the foundation of most major testbed federations in the world, including GENI and Fed4FIRE. However, there remain some testbeds that were designed and grew outside of this world, making different, interesting and sometimes better design choices. In this paper, we describe how we added support for the GENI Aggregate Manager to the Grid'5000 testbed, a major testbed focused on HPC and Cloud that was developed for the most part independently for the last 15 years. From this experience, we draw some lessons and recommendations that could help improve the testbed management ecosystem.