If you can't figure out the seam lines for regular piecing, foundation piecing isn't going to do it. I think I would go with piecing the squares, then applique the 2 horizontal and vertical elements. The only other option is to graph it out as suggested, then piece in small squares & rectangles. This will probably add a lot of extra seams, however. I'm not sure how that would look when joined to the center section you have already completed.