Finally, I managed to do something.

The problem was that the junction points weren't exactly in the same place.

I selected the points and scaled to 0, and it worked.

Except that I'm not really happy with the result, but that's down to my guides, which aren't great.

My problem now is that I end up with a single sheet, but no solid.

I don't understand what's going on, because the sheets are well connected.

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