The 3D scanning company Bellus3D has designed a 3d printable tool for masking fitting. They are known for a 3D scanning app for your face, which works with a cell phone. The app didn’t function very well, but a feature in the app “Mask Fitter” is quite useful.
The 3d printed mask fitter tool is designed to snap over your existing mask and allow for it to fit the shape of your face. It allows for a tighter more comfortable fit of your mask.
“A standard face mask is loose-fitting and does not provide a tight seal.
But, face masks are less expensive, more widely available and can be homemade out of different materials.
The Bellus3D personalized Mask Fitter frame is a new innovation designed to be pressed on top of an existing face mask. It provides a comfortable, tight seal for significantly improved effectiveness. It is for personal use.”
Bellus3D provides the standard fitter for free and a customizable version for $.99 US Dollars. A Premium version is also available for $2.99 US Dollars.

Conclusion
There have been many COVID-19 related prints in the Internet’s vast 3D Printing library. This is an unusual find which is well worth the time to print.