Great finishes all!!
I agree with the concept that you can add width without needing to add length. I often do this by adding side borders without adding bottom and top borders. I do like the look of closing it all up with a final border around all 4 sides tho, to kind of make it look finished. One of my favorite things to do, in your quilts case, might be to add the black between the rows and outside (3 strips) in a narrower width, perhaps 4", then do some piano keys on the outsides and finish with a 2" black border all around.