ReRe,
Do you have any fabrics in your stash that will coordinate with the quilt fabrics? Things that would look good next to it? I would take some of that fabric and add top and bottom borders equaling 8" at the top and bottom, adn then add an inch (1") to each side before layering, basting, quilting and binding. This should solve your problem.
If straight, solid borders don't appeal to you, you could always: 1) applique something to the top and bottom border; 2) use multiple borders that would equal 8", sort of like a mat and frame around a photo; 3) make your top/bottom borders have long stretches of fabrics that are broken up by blocks that are like the piecing in your center.
There are other methods you can use, but sometimes a photo will help those here on the board to assist. If you can post one, you may get even more help.
Also, you don't indicate if you have any extra fabric left from your piecing. That can be used, too, to your advantage. You've just got to think "outside the box"; you'll come up with something!
Odessa