15" is a bunch! Assuming your strips and 9 patches are 2" finished pieces, an extra row only adds 6" so you would need to add at least 2 more of those rows on each side. My first thought is to do the same as your first border but put the 9 patches in a different spot. Example - the next row, split it in thirds using 2 nine patches between the 3 strip pieces. Then do a third row using 3 9 patches separated by 2 strips. I'll try to show it this way = row 1 (current one) is 9 -long strip -9. The next row would be strip - 9 - strip - 9 - strip. Then the 3rd row would be
9 - strip - 9 - strip - 9. This would add 12" to each side and if you use a bit wider binding (ending 1") that would bring it to 13" each side. I can't think of how to increase it any more and keep a balance. If your finished strips and 9 patches are a 2 1/2" or 3", then problem solved, you will have the desired increase. Please be sure and post a pic of the finished top. I absolutely love the colors!