this is a selfdesigned mask with filters. I designed it for my little brother (9 years old) so you should scale it before you print.
I cushioned the edges with foam for more comfort and so that it is seales better. I recommend printing with 3 outerlayers, infill around 20 percent and support.
The filters are on the inside. I used cosmetic pads as filter (2 pieces). The cap gets screwed on. The air flows from the outside under the lips to the filter. This helps to prevent soaking your filters with rainwater.As for how to put on the mask: I used rubber (8 to 12mm works great). The upper rubberband is just one piece fastened on both sides, wheres the lower part is cut in two and adjustable with a velcro fastener.
There are different files so you can adjust the mask and insert the filter afterwards in your slicing programm.
This isn´t am medical inspected mask. It wasnt´t checked if it approves safety standards, etc. it´s just something homedesinged, so wear it at your own risk.