Sky portal ? best way to texture skybox on object

Hi,

I run into some trouble while making a project. I have a room which is supposed to be customizable in size and different assets (doors, windows). For the exterior world a simple skybox just look fine. But, what is the better way to create "holes" in the wall where the doors and the windows will be.

For me, there is two solutions :

1- Quite difficult : manipulate vertex and uv data to really create hole in the geometry.

2- A bit easier : Use render to texture to get the correct skybox perspective and project it onto somes plane (on top on wall) that will looks like a hole. This is similar to Valve's portal in a way.

So question is : Is there any better way to do that ? For solution 2 I try to implemanting and almost get the result I want but I lost my depth map while playing with shaders. This resulting having my hole even on top on object between the camera on the wall (which is not good)