It looks a bit wonky in this photo. I am going to draw a square on the flannel the same size as the top – so that I can block it before adding the wider outer border.
I measured the width across the middle and cut the strips accordingly. I had a lot of easing to do – because all of the blocks on the edges are on the bias. I handled the blocks as gently as possible when joining and pressing them – but there is no way you can avoid a bit of stretching. If I can be sure that the top is square at this stage it should be square after the outer border is added.