The only way is to use areaportals to seal those areas behind and avoid they're watchable

That's one of the elevators, right? If you kept the usual elevators shaft layouts, you should have a testchamber door right as you leave the elevator shaft, right? all you need to do is close the door behind the player as it leaves, and use an areaportal in that door and close it too as the player leaves (add some delay after the door has been closed so the player never sees it closed): use a thin brush right in the middle of the testchamber door prop extended to its sides making sure both edges of the areaportals touch world brushes, and regarding up/down also extend its edges so it touches world brushes. Like so:

Details within the skybox and outside in the 3D skybox have ambiance to give a certain state of realism.

Also geometry like this would be automatically hidden if you thought about the position of your leafs. Probably the easiest option to actually hide the elevator shaft is to place an areaportal within the doorway that closes visibility when the door shuts in front of it.

yishbarr wrote:
Sorry for bumping this thread, but does anyone know how to place an areaportal so this won't be visible? http://cloud-2.steampowered.com/ugc/558736503152417114/0BDBEFE477A9650FAE3A0CA9F5F4D064B3A9B2D3/

I'm guessing the skybox brush is right up against those squarebeams? In which case I'd just expand it so that there is space for you to texture that nodraw wall. Or even skip the first step entirely and just texture the wall.

That is assuming, of course, that it's a skybox issue.

yishbarr wrote:
Sorry for bumping this thread, but does anyone know how to place an areaportal so this won't be visible?

To be honest, I don't really know what it is that you need to hide... If you need the exit area to not be watchable through that window, just do as srs suggested and texture that side wall inside the window. It's all about changing the chamber layout to get the desired effect; for example, for that area, I would just push the window further than the exit door, this way, since there already is an areaportal at the door making what's beyond it to not be rendered, you won't see any "artifact" through the window... but I'm not sure if that is what you want to achieve...