Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4266555.
I've been making Montana Masks (www.makethemasks.com) for local hospitals and ran out of elastic. So I came up with this. You need to use TPU or some other similar flexible material. I'm using SainSmart TPU, which seems to work fairly well on my Prusa. Clips should be in something rigid, like PLA.
You'll need 2 clips and 4 straps for each mask, with one strap going through each loop hole on the mask, then run two opposite straps through the clip (into the middle slot, loop over and out of the outer slot). This lets you tighten the straps by pulling on the loose ends. To remove, pull the clip away from your head to loosen the straps.
Edit: Mask Straps Shorter is a shorter version which can fit 4 sets per bed with a Prusa i3. printer (210x250mm print bed). I also increased the size of the hole blocker so it'll fit with the SLO Respirator remix @ https://www.slorespirator.org/