Fit testing of this version was found to be air tight when covering the opening of the hard shell (no cover and filter attached). Fit may vary between person to person so further testing and adjustment is needed. It was noticed that the filter cover may not hold thin materials well so may affect performance. For thicker materials cut to 70x70mm, I have not yet tested due to not having the materials available.
Design proves the possibility in creating an air tight fitting mask using 3D printing. 3D printing isn't the most suitable production method, so I would suggest for those who have better CAD skills than me to try to adapt the concept to alternative methods of manufacturing.