Bought two samples of rubber and tested the full fixation version of https://www.fixthemask.com/

One was a roll from Amazon and the other was from Grainger.com searching for the product recommended on the plans.
The Grainger 12x12 sheets each yield 3 cut-outs and were clean and glossy. I bought the regular grade material as the military grade rolls were backordered.
The Amazon roll was rougher (like a pencil eraser) and left a bit of residue on my skin.
Both smelled like a tire store.

I wear a size 7 1/2 hat (so larger than average male) and I found the best fit was enlarging the download cut out plan to 110-115% on a photocopier. It was still pretty tight but the straps were thicker/sturdier
If you have a normal size head or want a very tight fit, then keep it actual size.

Trace loosely around the cut out. It's OK if the straps are a little thicker than the plan.
I used a sharpie to draw a faint line on the rubber. The rubber will destroy the tip of your sharpie after a few cutouts.

When cutting out the rubber, make sure every "corner" is actually gently rounded, leaving that area thicker. If you go to a corner and make a sharp angle with your scissors, the rubber might tear when stretched.

If the rubber gets nicked, some black vinyl electrical tape can repair it without losing too much flexibility.
I also tried clear silicone and it did not stretch enough and broke too easily.

Overall, it worked very well with much less leaking than a tightly tied surgical mask. It does not help much at the nose bridge, so make sure the metal band is bent to fit. The seal was a little better than an N95 with 2 elastic bands.
It doesn't get a paper surgical mask to N95 level for filtration, but for most casual encounters and flying, you don't need that level of filtration. You do need a better seal than an "ear loop" paper mask, and this does the job nicely and not that uncomfortably compared to the N95 with elastic bands that wrap behind the head.

Hope this helps.