I loved all of the solutions offered. Feline’s solution seemed like the best one and your finished product is the proof. I had been thinking of just a large square, which would have been the same size as the square that you completed with the two small squares. Your choice is better, because it retains your original border. I would be interested in seeing what a fan would have looked like.
But I have to say that I’m really amazed at the expertise of the various members who were able to change your original concept and show a rendering of their ideas. In fact, I’m in awe of their technical prowess.