Pleated style - six layers of 80gsm SBPP

Not so long to make. A 1m length x 19.5cm : 

I folded the 1m into thirds, then half again. 

Then I inserted the nose wire and sewed it into position with the side stitch.

Then a vertical line (long stitch) to bind the layers together

The I pleated one side with the stretch zigzag stitch - slightly inset from the edge. Then the other

Pleating meant one pleat at a time, raising and dropping the presser foot to get each one lines up. As you could see I didn't manage to line them up left to right.

Then I trimmed the edges, and zigzag'd them to tidy them up.

next was knots in 19cm of elastic (two of these)

Last was tight zigzag stitch to anchor the ends of the elastic on the outside of the mask.

480gsm of SBPP is about 92% filtration effectiveness [DATA]. There are edge leaks though so it'd be less. The fix-the-mask gizmos would aid this type of mask, but I don't have one to try. If you're wanting washable F.E above say 85% you should choose another style of mask perhaps.

The SBPP used: £0.25 (UKP) at retail prices. The nose wire was upcycled ring-main wire (copper in plastic sheath - 11 cm or so).