Could you carefully cut the yellow part of the duck out and re-make the applique out of another yellow, leave and turn under an eighth of an inch? When you are satisfied it fits inside the satin stitch, apply an iron on to the back and carefully, maybe with one of those small applique irons, affix it in place. Then do a hand applique stitch around the duck, right up against the inside of the satin stitch. You might just want to iron the middle of the duck to hold it in place so you can pull the fabric right up to the satin stitch, and then iron the rest after you've done the hand work. If you are careful with your stitching not to go completely through the quilt, it won't show on the back.