Starting a scenery window box

The path of my around the room layout passes across the middle of a window and that's where I am working right now.  I want to still be able to use this window sometimes for ventilation but still have some background scenery behind the tracks.

So what I decided to do was to build a sort of window box that will hold a continuing of the background scenery and can be lifted up and out to access the upper part of the window.  The window can still be opened from under the layout even when the box is in place.

Here is the window box a little further along with a coat of primer and almost ready to start adding some scenery.

To be continued.