Since you showed us the illustration on how you cut the outside triangles, you will be working with a bias edge. I feel this requires sashing. Measure the sashing strips accurately and then pin the block to the strips. When you feed same under the sewing machine needle, just make sure the sashing strip is on top and this will assure the bias edge will not stretch. If the outside of the block stretches, you will have a terrible mess and all of your work will be for not.