Summary

I made this adjuster for round, 1 mm elastic for the earloops of a cloth mask, but it could be useful for any small strap that needs to be adjustable. The .scad file is reasonably parametric, so you should be able to adjust the parameters to accommodate a different type of strap.

The holes are subtly bigger on one side to help make threading easier, i.e. on either face, the right edge of the top hole opens wider than the left edge, and the left edge of the bottom hole opens wider than the right edge. (This is true regardless of how you turn it.) So if the strap is very snug, try to thread going into the large side and coming out the small side and it should be easier (see diagram).

Designed with OpenSCAD.

Sewing instructions for the mask pictured are available at: https://diysurgicalmasks.wordpress.com/2020/03/21/sewing-pattern-for-reusable-mask-with-disposable-insert/