Cheap (less than 50 cents a piece) ebay KN95 masks fit a lot better than surgical masks, and the wearer is more easily heard through them than through a cloth mask (at least according to my small single-blind test in the family), which is important as I am a (humanities) professor. I read that the N95's inventor, Peter Tsai, has suggested a weekly airing and reuse schedule for N95 masks, letting them self-disinfect by drying out the viruses, so I have a rack with masks for different weekdays.

But the cheap KN95s have some downsides: the ear loops can break off, the fit is loose, resulting in airflow around the edges, which decreases protection to self and others, as well as fogging of glasses, and the nose strips can break after a while. So if I was going to reuse them, I had to make them better.

What I did was:

strengthen straps
turn ear-loops into more secure, snug-fitting behind-the-head straps, though still relatively easy to put on and off due to 3D printed clasps on the bottom strap
replace flimsy metal nose strip with a solid plastic 3D printed version. The solid plastic nose piece also lets one adjust the mask by holding the strip without touching the potentially virus-infested mask material, and one can wipe it afterwards.

The result is a much better fit, with much less of the tell-tale feel of air slipping in or out at the mask edges. I had a bit of occasional fogging of goggles while playing racquetball, but I was breathing particularly hard I expect, and there is some fogging even without a mask. I don't know how it compares to N95 (and indeed, I wouldn't rely on them in high congestion situations; I use N100 or P100 respirators--with valves covered--in those cases, but my audio comprehensibility in those respirators isn't as good).

For those who don't have a 3D printer, I will suggest some alternatives to some of the steps.

Supplies

KN95 mask
assorted rubber bands
Shoe Goo glue
PLA filament

Step 1: Strengthen Straps

The straps look welded to the mask body, but the welds are weaker than they look.

Put a smallish glob of Shoe Goo under the strap just where it comes out of the weld, and a larger glob covering the weld and sticking out some distance on the mask. The larger glob was about 1/2 inch in diameter. Allow to set for a few hours before the next step.

Step 2: Behind-the-head Straps

Cut the ear loops with scissors. I've seen masks with two different ear-loop lengths. If you have the long ones, you should be able to just cut them in half. If you have shorter ones, cut the ear-loops in such a way that the bottom part of the strap is a little bit longer.

Now, put clasps on the bottom straps. I made a simple 3D-printable design that is easy for me to fasten and fasten behind the head. You can just tie them to the straps. I've been surprised that because of the cushiness of the straps, a single overhand knot, cinched tight, does not appear to come undone. Do the knots fairly loosely at first, and fit to your head to ensure that the fit is as snug as can be without being painful (or likely to become very painful after a couple of hours). If your bottom straps are too short to meet behind the head even with the clasp, tie a small rubber band between one or both clasps and the strap.

If you don't have a 3D printer, the following worked: take a medium sized paper clip, fold it with pliers into a hook shape, and tie it to one strap's end. (If you want to be fancy or are worried about being poked, you can put heat shrink tubing or electrical tape over it.) Then tie a rubber band or some other loop on the other strap's end. I haven't actually tried this beyond a quick fitting, but it was easy to attach and detach behind the head, and it wasn't uncomfortable.

Then tie a rubber band between the two top straps. Again, you will want to experiment exactly on the size of the rubber band (or combine two of them) and where exactly the knots go on the straps for comfort and good fit.

Step 3: Superior Nosepiece

This is the trickiest. There are multiple options here. They all start by removing the old nose strap, as carefully as possible.

No 3D printer: I recommend using some stiff solid copper core insulated wire, folded twice. I've used such wire (salvaged from a broken fluorescent lamp, but you can also buy some at the hardware store) in cloth masks, and when doubled up, it keeps shape very well. Remove the metal strip, and glue on with a generous bead of Shoe Goo, sufficient to cover up any damage you did the fabric when removing the old strip. Maybe you can use clothespins with ends covered with painter's tape for clamping.

3D printer: Glue on a solid plastic nose piece, again using Shoe Goo generously enough to cover up any damage from removing the old nose strap. But you need to design yourself a nose piece, since everyone's nose is different. Here are some options:

download the OpenSCAD files for the two nose pieces I designed, and tweak the numbers going in the Bezier curves to match your nose, then print a prototype, tweak measurements again, and repeat until satisfied; my own nosepiece took seven prototypes
fit a stiff wire to the bridge of your nose, scan it or photograph it with a ruler for scale, trace it with Beziers in Inkscape, scale it to the correct size, and import into 3D design software; I did this with my son's nose piece, using OpenSCAD and my svg2scad script; I recommend reducing size before printing for better fit
print one of my nose pieces in PLA, and while it's still hot and soft from the printer (or after you've reheated it with hot water or something else), fit it to your nose bridge, being careful not to burn yourself; then let it cool.

Designing a nose piece lets you 3D print multiples quickly.

Finally, if the nose piece doesn't quite fit your nose after gluing it on, you can heat it up until it's soft, being careful not to melt the mask material, and reshape it a little.

Note: Before I dialed my prototype in fully satisfactorily, I found that gluing a rectangle of rubber (from an old bike inner tube) with Shoe Goo on the inside of the nose improved the seal. Using rubber strips to improve seals in general is something that may be worth experimenting with the in the future.