Since you have already cut out the squares and rectangles, there is one method I heard of that may help. After you mark the diagonal line on the square, pinned it to the rectangle and sewed on the line, you press the square back, but you DO NOT cut the excess off. I don't remember the name of the person who came up with this, but she said that by cutting the excess, you are cutting off the part of the rectangle that gives you the correct size. I guess you could cut off the excess of the square and leave the rectangle whole. That would eliminate some bulk without losing the size.