Do you have enough of any of the fabrics in the quilt for borders? I would not introduce another fabric unless it had all the colors in it. I would put a 1 inch border of one of the fabrics, followed by a border the same size as the squares. I usually lay the fabrics with just the amount I am thinking for the borders showing. I leave it where I can see it often. Usually within a day I know if it will work or not.