Adapted the "N-Scale Magnet Coupling" to fit the Roco-Couplings.
Single magnet coupling: 3mm diameter round magnet with 1mm thickness.

Should be printed with a SLA/SLS-Printer for best results. Did work with my FDM-Printer with layer height 0.04mm and supports, but had some post processing to do.

Works fine, but it's still a work in progress.