Hello @guiomar,
to plot topographies of PSDs, please have a look at how mne.viz.plot_epochs_psd_topomap() does it (please follow the link to display the full block of code I highlighted for you – the below preview in the forum is truncated):
It’s really not much code and I think it demonstrates precisely what you need, without having to fiddle with montages or manually needing to extract data from the figure post-hoc.
Best wishes,
Richard