Yet another clasp for facemasks

Here is yet another clasp for mask straps. It should work for elastic, but I am using it with rubber bands.

I printed in PLA, with fan on, and doing two at a time. Doing one at a time made the knob become misshapen and melted due to the overhangs.

Backstory: I found that I could greatly improve the fit of cheap ebay KN95 masks by equipping them with a [customized permanent plastic strip](https://www.thingiverse.com/thing:4581977) in place of the flimsy wire and by switching the ear loops to around the head straps. For the ear loop mod, I strengthened the bond between the loops and the mask with Shoe Goo, cut the loops in half, and joined the halves with rubber bands. But when I made the fit really snug, it made the lower strap hard to put on, and hence my need for a clasp I could put on behind the head.

The strap attachment part of these is inspired by [this](https://www.thingiverse.com/thing:4245317), and the joint by [this](https://www.thingiverse.com/thing:4285139).