How about doing applique numbers? It looks as though some of the letters are bigger/thicker than others. You could use that principle to do the numbers, since there are an uneven amount (10 letters for 4 corners). For example, upper left corner could have medium size 1/2/3; upper right corner could have larger 4/5; lower left corner larger 6/7; lower right corner medium 8/9/0. Or...you could use the corners for personalization--name, date of birth, etc. (if for a baby). Please show us a picture of the finished quilt!