I use a similar design, I love how simple it is.
But my last step is different: after folding the top + bottom to the middle and make the seam, I simply fold the sides twice to the middle to make tunnel. That is fixed with a seam and the mask is finished!
On top the finished mask 'in use', left bottom the tunnel for the band or elastic, right bottom the mask waiting to be used.
Have a !BEER