If you are appliqueing the cross onto the pieced background, I would suggest that you fusible to put 2 pieces of white together so that there would be no colors coming through (I speak from experience on this). As others have suggested here, I might audition 1/4" fusible seam binding to see if it might be too much, or whether a very narrow zig-zag stitch in black rayon thread might work. Not to be confusing, but I would add a narrow 3" purple border, or use none at all, as your quilt is fantastic. I'm sure that he will greatly appreciate this gift and will keep it with him always.