There are models of glass frames in the models browser (like the ones that are for observation room windows) but how can I extend them? Are there other models for that? Or do I use a brush?

Place multiple, small models of the glass to form a big pane, like they do in this map.

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Alternatively, you can use brushes.

You can use anything that fits, really. It doesn't matter if the frames are models or brushes, basically, if it looks nice as a brush, go ahead and use that instead.

Also a side note, I think it's better for you to create a mega-thread of questions as opposed to creating a new thread for small questions like these.

Oh, got it, alright, thanks a lot!