This serves the same purpose as the Surgical Mask Tension Release Band  for Ear Comfort & Extended Use submitted by Davis Becker (Model ID 3DPX-013410), but uses two pieces of elastic for infinite adjustability; the two elastics are separated by an angle of 100 degrees to allow for more stability on the head and hopefully prevent the mask from sliding down the face.

It allows for infinite adjustability, albeit it requires elastic (or substitutes) in addition to the 3D printed parts. There is a version for 1/2" wide elastic and another for 1/4" wide elastic; other widths are easily created. I have printed and tested the 1/2" wide version to insure it holds the strap securely and provides good fitment with my homemade mask. A pair is required for each user: the mask end is copied from the above referenced model - the mask straps are attached here. There are two elastic straps required for stability and comfort. The strap is inserted from the underside, wraps back 180 degrees and goes under the flexible tab. The tab allows the strap to be pulled tighter, but not to loosen (to loosen requires also prying the tab up while pulling the strap back). This makes it quick to tighten. Having two straps helps keep the mask from slipping down the face and make it more comfortable without pressure on the ears.

There are alternatives to elastic strap should that be in short supply
1) Binding tape (sewing).
2) Two thicknesses of cotton sheet (shown in photo). This was cut from the finished edge of a bed sheet.
3) Flat shoe laces
4) two layers of cloth medical tape (sticky sides together).

This was printed and tested on a Makergear M2 FDM printer using PETG. Because it is only 1.6mm thick (8 layers on my machine), I suggest these print settings:

Solid Top layer: 1
Solid Bottom layers: 1
Perimeter layers: 2