After printing some prototypes of the Montana Mask, I noticed a few geometry issues in the original model that could cause defects in the final print. After a week of tweaking, I ended up rebuilding about 85% of the design from scratch in Fusion.  Key changes;

• Modifications in model symmetry to fix defects. Example: Seam under the chin.
• Adjustments to nose and chin shape for improved comfort and fit.
• Reworked strap loops for added strength when using different materials, and to facilitate printing without needing supports.
• Added simple grille to help protect filter from physical damage, and prevent wearers from putting the filter in backwards (I've pictures of people doing this unfortunately).

The only thing I haven't modified was the original Montana filter flange which I've included.