MacBook: Prevent mouse from switching to external monitor placed above main display
May I suggest simply changing the arrangement of the screens (in the dialog seen in your screenshot, not physically).

You can either place the screen to the side, or even just place it at an offset. Say you place it as far to the right as possible (while remaining above the main screen). In that case, you could move the mouse from one screen to the other by going all the way to the right edge of the screen and up.