How about one relatively narrow solid border in one of the darker fabrics, followed by a pieced border from the pattern. You could take a slice down the center symmetrically (however wide that would make it) and repeat it with the same colors. Depending on how the corners come out, you could continue the pattern or have squares in the corners.
When I get in a crunch like that, I take grid paper and actually draw it out. It gives my inner 5-year-old a chance to play.