I've made a couple of duvets. I don't do the button hole thing. My method is not so special. I buy spools of narrow satin ribbon (in coordinating colors). In the places where a good and patient quilter would put their button holes & buttons, I attach ribbons - one to each side, and march them across the entire opening. I use my sewing machine's Circle Stitch and secure them by their centers. Then, after the quilt or blanket is inserted, I simply tie them all closed.