This is respirator I use during the outbreak of Covid-19. It doesn't require additional montage, except for addition of a filter and straps. Shoe laces work quite well as straps and you can read more about suitable filter material down in this description.

Don't forget to disinfect surface of the mask after each use.

More about home-made filter selection here:
https://www.researchgate.net/publication/258525804_Testing_the_Efficacy_of_Homemade_Masks_Would_They_Protect_in_an_Influenza_Pandemic

Print instructions

Can be printed without supports!! Though I recommend to use raft for additional adhesion to the build plate.