Building a stud wall around cornice
The only downside to your approach is the lack of rigidity in the upper end of the frame because of the multiple joints. Once the framing members are secured to the walls and ceiling, this should be largely resolved.

You could strengthen the frame by adding a horizontal member right below the inverted corners, making a large rectangle with a smaller one above it. This should be well above the door frame of the wardrobe.

Edit by asker-
Image of my interpretation of answer: