What a cool idea! You could get the words embroidered on fabric or you could get them appliqued on. You can add iron on photos of the two of you. You can do any pattern with nautical fabric so you would be able to use an easier pattern. A great way to keep the secret is to hire someone here on the board to do embroidery with the question. Then you can work on piecing the top of the quilt with a relative, not revealing the proposal block until you are nearly done piecing the top (just letting the quilter who is helping know what size block will be added.