OK - it's a long way from a finished thing. The gyroid separates the framework from the empty space. Framework just shows nodes, not mid-points. And all the dots need to be joined. And then I need to render just the framework and compare it to the crystal structure to make sure I actually do have the nodes in the right place. Then I need to translate the framework to the other side of the gyroid and figure out a way to replace the frame with a space-filling blobby thing. But even so, it's better than I expected at this stage!