First version of this tutorial (sewing without elastics) is available here.
This mask was created using the pattern provided by AFNOR (French Standardization Association) here :
I added an opening at the bottom to have a possibility to insert a filter, just like if it was a pillowcase.
Unlike the mask with the central stitch, this model is less fitted and as a result a little less classy. But having sewed, worn and offered both, I finally find this one more comfortable in the long run: it's a bit like a difference between wearing a tight pencil skirt and a cotton hippie dress =) And since once outside it's better to not to touch the mask, I find this model more versatile: you can laugh, talk and even scream without having to adjust it.