It appears that the width of your quilt is about 50" wide - a full strip minus one diagonal square, plus some additional. There is no reason you couldn't buy more of the same fabric and add more to one end. You'd need 6-7 strips for each 5 that you want to add. To get the randomness, you'd want to start in a random location, cut the strips to the right width (with a little extra to allow for errors), and mix them up when sewing together.