Will the S be appliqués onto a large centerpiece? You could do that with a white background, that is used for signing. Then LOTS of persons could sign in the open space around the S. If you use squares your signatures will be limited to the # of squares you have room for.
You could appliqué the S onto the white. Know the size you need and cut to fit so people don't sign outside the lines . Maybe add and inch tops, and duse a disappearing marked to frame your sewing line. Iron this all to a large sheet of freezer paper, to make writing easier. By doing this, your signatures will be all over the place but, many can send wishes. You can then make WIDE boarders with your novelty prints
prayers for her recovery