The new improved version of this tutorial (with the elastics and the insertion of a nose clip) 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. Second difference is in the attachment system, which allows to fix the mask using only one knot.
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.