Black squares on main screen

Finally getting close to a final monitor configuration. I have moved my thrustmast MFCDs onto my primary monitor because its 4k /39" so I can spare the realestate, and the performance gained by running on 1 monitor justified it.

However when I set the size so the buttons align, they don't quite fill the square. Also I ahve expored my RWR and it has no background at all.

I read an old post about creating a black square by using negative width/heights, but when I tried that it looked like a reverse image of some opposite part of the cockpit, especially when I panned around.

Challenge, if you choose to accept it: How can I black out a rectangular part of the screen, above the "world" drawing, but below the MFCD/RWR.

I would prefer to remain in fullscreen mode for VSYNC reasons.