There are a number of synthetic rubber compounds which can be made from domestic petroleum supplies, to make straps. Tear resistance and tensile strength are poorer with most (esp. the cheaper compounds), however, those issues can be addressed. And, as you say, elastic is not a mandatory need for straps, although I consider it “better”.
The tough part with N95 masks is the media, and consistency of the media. That takes some fairly sophisticated, hard-to-get / expensive-to-get-it-right equipment, and good QC. There was a good article on this, on here, a while back. China has a lot of trouble with both.
Thanks - and it’s not quite WW2 again. Malaysia would probably have no issue selling us the rubber to use, if we were able to get serious about actually making them. And yep, the media is critical, way over my head, but who knows, maybe 3M can pull it off, and could license it to others.