I tend to add borders even if it doesn't call for them. Since you don't have enough of just one fabric, how about strip piecing what you have, then cut them down to what ever width border you want? If you don't want to do mitered corners, just add a square of one of the fabrics at each corner. This way you'll use up what you have plus have a border. You already have a sashing of black that separates the blocks. I like using up what scraps I have of the fabrics in the quilt. Scrap breed overnight.