OK, the next session is coming soon, so I am in a hurry (so laaazy, am I).
Here are my last try at this *@%%§§ Map :

Version with no transparency at opening on the central shaft :
Click image for larger version. 

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And then with transparency revealing the openings :

Click image for larger version. 

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And the Objets to hide an theam reveal the areas in MapTool :
Click image for larger version. 

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ID:	24366Click image for larger version. 

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ID:	24365Click image for larger version. 

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Is it better?

I won't be able to change it prior the next session but I nonetheless will be glad to learn more...