I think a thin black border would look very nice. If you don't have enough long pieces of the stipes to go after the thin black border, do you have enough of the other fabics to make 4 more blocks? I have used blocks from the inner pattern and carried them out (one in each border)to the outer border. You just make the wide border fabric the same width as the block and make them a design element. It looks like you planned it that way and you can seam the border where the blocks go in.