Materials: Smooth-on 940 food grade silicone rubber. Silc Pig Colorant (without a bit of colorant, the silicone is pink) Silicone release spray Elastic bands Filter material

Equipment Reusable caulk tube: 'Tube A New' on Amazon is probably the easiest to clean. Caulk gun Vacuum chamber Pressure pot
Reverse pliers Hot glue gun. Angle wire cutter Scalpel

This is a 3 part mold for making a silicone mask. It is held together with 1/4 inch bolts.
Some hot glue to cover the cracks between the parts of the mold will reduce the silicone that may leak out.

This mold will make a mask with the size Large. The mold should be scaled down if you are trying to make a smaller size. My best guess is 96% for Medium and 90% for Small.

The mask and 2 inserts fresh out of the mold and without the flash trimmed off, will weigh about 6 oz. I would mix a little more than that at the start. Maybe 7 or 8 oz. I prefer to have a little too much silicone than not enough. Any extra silicone can be poured into plastic lid for a food container to make a simple coaster.

The parts of the mold should be smoothed down if you are trying to make a finished product. This will make a better looking part that is easier to clean. If you need to make multiple part, some of the silicone may soak into the 3D printed mold. If all parts are the same color, I wouldn't worry about it. Otherwise I would go from lightest colors to darkest. Or use something like Smooth-on XTC‑3D or a light coat of lacquer. The silicone should be degassed in a vacuum chamber and then poured into an empty reusable caulking tube. The caulking gun lets to apply slow steady pressure to push the silicone through the mold.

The mold is orientated so that the mask is upside down.
Slowly inject the silicone into the mold through the hold in back that comes out near the nose piece. Holding the mold at a 45 degree angle will help minimize bubbles. The silicone will come out the vent at the top to show that the mold is full. The wider holes around the vents are reservoirs for a little extra silicone. If there are any small leaks or bubbles, this extra silicone may be pulled back into the mold.
When the mold is full, plug the inlet with a 1/4 bolt wrapped with some paper towel.

Place molds in a pressure chamber to minimize bubbles. I use a converted 5 gal spray paint pressure pot.

In a heated pressure pot, the silicone will take about 2 hours to cure. If the pressure pot is not heated, best to leave it overnight.

Opening the mold should be done slowly and carefully. With the reverse pliers, you want to apply just enough pressure for the silicone to pull away from the parts of the mold. Work your way around the mold to slowly loosen the silicone's grip.

Angled clippers and a scalpel can be used to cut away any extra flash.

I recommend using 2 layers of melt blown polypropylene as the filter material cut into 3.5 inch squares. A water resistant layer on the outside will help repel any water droplets in the air. The mask is help in place with two elastic bands. Each is 24 inches. There is a slip know on one side to make it easy to adjust.