So it's 64" wide now ... right?
I agree, you need more color if you're going to make it at least another 20" wide for a King.
I would actually try a border that combines all of the colors plus the B & W print.
Braid?
Several squares set on point?
Opposing triangles made from strips?
Either way, I would combine color with B&W and use a diagonal but linear pattern. The linear pattern will compliment the log cabins, and the diagonal will bring the focus back to the center which will be important because your border will be so large.