I would do a simple 4 patch for the centers and since you are hand sewing them in it shouldn't be too hard. Carefully press around a four patch area to determine the size of finished block you will need. Put the 4 patch behind the intersection and kind of appliqué the surrounding fabric to it. it will give the same appearance and won't be as hard as sewing them in. Good luck and you will have a treasure when you get it done.