I have tried for a long time but I just cannot fill that surface (black arrow). I understand that the problem (presumably) is, that the endpoints are not on one surface. But I cannot find where.
PS: this will be a 27 seat small theater in a resort on the Island “Koh Mak” in Thailand. One night for free for the problem solver 
The edges bounding the opening are not planar. Draw a diagonal to create two triangles to get the faces need.
You have a number of reversed faces. You should correct those and you should be doing that before you start adding materials.
Since the seats are basically identical it would be more efficient if you would make one of them a component and copy that instead of using groups.
I was too slow.
That was a VERY quick help. Thanks.
For me as a beginner: what did I do wrong to end up with non planar edges?
PS: DaveR an Box both one free night (once we open and once you are allowed to enter Thailand again)
I’m not sure you did anything that would be called wrong. Sometimes not all the edges will lie in the same plane. I don’t know if your model is drawn incorrectly or if you are just running into one of those basic geometry things.
Thanks for the offer.
Not sure if this is exactly what you want but I’ve converted the elements to components. I suspect there needs to be some modifications but maybe it’ll work as is.
thea2.skp (210.7 KB)
It often helps to build these things using follow me. You can try different profiles and number of sides to get something you like, then you can break that down into a couple of components that you can array.
Hello Box. How did you remove that line? If I draw it, it fills up but when I delete it, I lose the surface…
Hold ctrl with the eraser tool. The edge has to exist but it can be softened, smoothed or hidden.
