if the box is far enough away from the rest of the map, you could make the box a prop with a short fade distance ... the only issue there is players would still be visible in the box (maybe fixable with an areaportal?)
edit::
(assuming the box is up in the sky)
what you might be able to do is make the outside of the box the skybox texture and make the inside an invisible texture (or a 1 sided mesh prop? or displacement?), and then inbetween the outer layer of the box and the inner layer, make areaportals with a fade distance (so players far away from the portals cannot see through them)
This is all theoretical, and probs wont work
Your edit is almost exacly what I've been using this entire time. The room is indeed high up in the sky and I just needed to know if there's a better way of accomplishing what I have in mind. I remember doing all the outside a skybox texture but it wasn't working properly (don't quite remember why, I think it was because players from the inside couldn't see anything on the outside).
What I have been using so far is a box with 5 walls with a skybox texture on the outside and the walls-and-such textures on the inside. (for this next part, keep in mind that I was a begginner when I was doing it so please don't laugh at me
) The 6th wall is a func_detail with a skybox texture on the outside and a glass texture on the inside. Right outside of the 6th, glass, wall there's an areaportal, which is closed by default. It's being opened by a trigger_multiple, which covers the entire room, when a player enters the room and closed back again when everyone leaves. So when somebody's in the room anyone can see it, but when it's empty it's nice and completly invisible.
I was trying to make the fading areaportals work, but it didn't work for some reason.
If you want to see this in-depth I've built a tiny test map:
WMF File,
BSP File
The room itself is suspended right under the top brush. There are props around to test the see-throughness as well as inside (along with a func_detail and some world brushes) to test its secrecy.
The info_observer_point let's you see how the room looks like from the inside when the areaportal is closed.
Thanks for the answers, guys!