UPDATE 1: 06/18/2020

Added an updated locking ring with a spot for a filter to be added. Also added a locking ring cap that can be used in place of the original locking ring or as a cap for the filtered locking ring.

This is my take on an exhaust vent for adding to either rubber/TPU/plastic masks or fabric masks, if you want. I hate having my face heat up because of wearing a mask, and I wear glasses from time to time, and they always fog up when I have a mask on. For this vent, cut a 25mm hole in your fabric, and thread this through, then tighten the locking ring to the fabric. Depending on your fabric, you may have to cut a smaller hole and stretch it around the barrel. Add the TPU valve seal (a.k.a. rubber disc) to the center peg. The disc is a very tight stretch, but it fits if you are patient.
This is intentional. This is modeled after the exhaust vents used in U.S. military (Avon) M50 gas masks.

I printed the body in PLA at 200 degrees with 40 degree bed. I know, PETG would have been better, but my wife wanted them black, and all I had in black was PLA. The TPU disc was printed in, well, TPU. I used Priline black TPU at 220 degrees with a 70 degree bed. The TPU was printed at 0.12mm resolution for 3 layers. I might even recommend 2 layers, just for better flexibility in low airflow situations.

The threads are SP415-M-8 (soda bottle) threads. They seem to be a bit sturdier, and they work are designed for PET bottles, so why not.

I may design a front cover for this to sort of hide the TPU disc, so keep an eye out. I'll add it to the project if I do.

This is my first upload of a design I did myself, so be kind with the comments, and if you have suggestions, I am happy to receive them. Otherwise, please enjoy!