For new light sources, like X-ray free-electron lasers (FELs), highly precise diffraction-limited optics are needed, which are leading to ambitious requirements for the X-ray mirrors used in those facilities. For appropriate beam focusing and alignment, a control of the shape on the single-nanometre precision level is necessary, which generates high demands on the manufacturers and on the metrology. To face these questions, the project MooNpics – Metrology On One-Nanometer-Precise Optics was established. With a European-wide round-robin test, the goal is to push the frontiers in mirror metrology in Europe to single-nanometre figure error precision. Within the MooNpics project, a special mirror holder for long X-ray mirrors was developed that provides reproducible and well defined mounting conditions in each participating laboratory. The goal is to understand mechanical and stress influences and hence to improve mounting methods. Before the actual start of the roundrobin, measurements were done to investigate the influence of the mirror mounting. A procedure was developed to reduce the induced stresses and increase the reproducibility with regard to the planned round-robin.