If you have a long nose, you might be familiar with the pains of wearing a face mask, as in:

medical masks, as is now mandatory where I live, with the ongoing 2019-nCoV epidemic
PM2.5 masks, if you live in a polluted city

Not all masks allow for a tight seal in the crease of the nose for all face shapes. They let air blow into your eyes, and cause discomfort, fog on your glasses, and possibly conjunctivitis. A bad seal also renders the mask ineffective, in the case of eg: N95 masks.

I've finally set out to address the issue. And no, I'm not cutting my nose.

Before diving into this instructable, if you have light clay at home, you can try molding it around your nose directly. (adding a bit of elastic band or fabric might help it not break apart). I did that, and the result kind of worked, but looked ugly.

Also, if you're struggling with PM2.5 masks, try the 3M 9322 (3-fold) first. They fit me quite well. (Disclamer: I am not affiliated to them in any way).

Supplies

I've used objects laying around my house, including:

Alginate and plaster, plus a silicon bowl and spatula from an dental impression kit. The quantity for one impression was enough.
super light clay, stolen from my kid
10 cm of cloth elastic band, ideally white. Mine was 15mm wide. A strip of light fabric would probably work as well.
a hot glue gun
white (wood) glue
cardboard, clamps, cissors, water, and a sheet of toilet paper

Step 1: Prepare the Cardboard Frame

I've cut out the shape of my nose, more or less, in a sheet of rigid cardboard.

I've then hot-glued a wall that somewhat follows the shape of my face.

Alginate is quite forgiving of small holes, it will not leak so much. However, it needs a rigid structure to keep its shape once unmolded.

Also, be careful not to pinch your nose with the cardboard. If you do, the molding will work just fine, but the resulting nose piece will make it more difficult for you to breathe.

Step 2: Pour in the Alginate...

This is the messy part. Be very careful, as once you've got alginate on your cloth, don't expect to get them clean again. I did this step bare-chested.

Alginate hardens in a matter of seconds, so you need to act quickly. Also, you need one hand for holding the cardboard, and one for pouring. If you spread it too thin, it won't hold once unmolded.

Having someone help you, or working in front of a mirror, might help.

Step 3: Pouring the Plaster

Once you've got your nose cast, you're ready to make a plaster positive. Do it right away, otherwise the alginate cast might have time to dry, and shrink.

I used a sheet of plastic from the cover of an old notebook, which I clamped to the bottom of the cardboard frame. I used the spatula to first clean up the alginate that had oozed below.

I laid the plaster mold sideways, so it wouldn't spill out, and left it to solidify for half an hour or so; check your plaster's instructions.

After that, I trimmed all the concave bits at the edge of the cast, to ease the next step. I also had to remove one or two protuberences from air bubbles in the alginate; they got off quite easily and didn't impact the end product.

I ran out of plaster during the process, so I put a few small objects in the (big?) nose for filling.

Once you're happy with your plaster nose, you can discard the alginate impression, as you won't be needing it anymore.

Step 4: Final Step...

My first clay nose piece got stuck to the plaster, and I had to rip it off bit by bit. I then added a sheet of wet tissue on the plaster nose to ease the unmolding.

The second attempt failed because the bridge was too brittle. I fixed that by adding a small piece of black elastic band. I first massaged the fresh clay inside the fabric carefully on both sides, to make sure it adhered well. That worked like a charm.

The next attempt failed because the nose piece was too big (that's the photo with the elastic band. Don't do that.) It held on my nose, but it would not fit inside the face mask.

The last attempt worked allright. You really want to make the nose bridge as thin as possible. And the rest as well: all you need is to reduce the crease of your nose, not eliminate it completely.

Also remember that light clay takes a long time, sometimes days, to dry to the core. Start with something thin, let it dry, try it out, then add material where needed. Putting the clay in front of a fan speeds the drying a lot, but not as much as working in layers. The final nose piece picture here was made in 2 layers.

Step 5: The Result!

The nose interface piece feels at home on my nose, and doesn't pinch it, though if falls off readily if I move my head.

I ended up adding a little bit of double-sided tape, though the first brand I tried didn't stick to the light clay at all.

When I put on my mask, I fiddle with it until my breath stops blowing into my eyes. Voilà!

Wearing the nose piece notably improves comfort and airtightness of my medical masks (even though, I know, this type of medical masks aren't designed to be completely airtight).

As for N95 masks, I was already getting a tight seal with the 3M 9322 (3-fold) masks. I had a try anyways, and, while slightly bulky, the nose piece still fits in and allows for a tight seal. Other masks, like the 3M 9003 (which did not fit me at all without the nose piece), work better as well.