How about a white border the width of the squares in your last row now. Then 3 more rows that in each corner make it look like the corners of the log cabin blocks. Don't bring the colors from these 3 rows all the way to the center of the sides. Leave some white area. But step back each colored row as in the log cabin. If that leaves too much white space on the sides, you could also do those 3 colored rows at the center of each side, also stepped back as in the corners. Sorry I don't have any examples of what I'm trying to explain.