After printing, glue in a 12x3mm magnet into each side of the clasp to allow them to magnetically clip together and apart. I used these magnets: https://www.amazon.com/gp/product/B07DD5SLLJ/
To install the mask, pass the elastic through one of the two holes on each side, wrap it around the hooks and pull in the slack.
For a better fit, use a heat gun to gently soften the thin straps and bend them to conform to your head. Be careful not to heat the magnets or deform the main clasp body. It doesn't have to be a perfect curve as the printer plastic will have a bit of flex naturally.