Geomag doesn't come with decagonal panels, so they must be approximated with pentagonal cupolas. There's a rather surprising choice to be made when building this model: which way do I orientate the cupolas? The way shown in the picture, with the cupola's squares joining is the only real choice, because the triangles are slanted at too tight an angle for regular Geomag bars to be used. TODO: what is the angle? (with derivation).