I agree with the others that this quilt calls for straight line quilting, but rather than the box within a box that a true echo would give you, I would parallel each side, so that the corners are cross hatched and bring the sets of tripples from the sides across. With the minimalist nature of the quilt pattern, I agree that a gray "invisible" quilting thread or a pale grey might be a good idea. be sure to audition it to see what you like.
You have done an excellent job with this. I don't think I would even have attempted it. Be sure to show it to us when you've finished.