2 part TPU strap for the Prusa Protective Shield. Should fit most size heads.
Depending on your TPU hardness, you may have to shrink or grow the Z axis. Softer TPU should be printed thicker, while harder TPU should be thinner. The current thickness works for my specific TPU (Amazon basics white) at .2 layer height.
The half with the "pins" is thicker and not made to stretch much. The half with the holes is thinner and should stretch to insure a good fit. I went this route since it's a compromise between durability (thick half) and stretchy (thin half.) In theory only the thin half should wear out over time and need to be reprinted.
Keep in mind that some hospitals may not accept TPU as a sterilizable part. TPU can not be cleaned with bleach or be autoclaved. It can be sterilized with Isopropyl alcohol, hydrogen peroxide, dry heat, or UV-C. Be prepared for them to only accept this as a disposable, single use part (meaning you'll have to print a bunch per mask.)
That being said, if no elastic or rubber bands can be found, this is an alternative.
Update - XL version added that gives 40mm additional total length.
The author remixed this model.