The mane problem with the cord locks has been the slot for the string to get in.  That causes 1 direction to have half the grip of the other direction.  The string can get out of the slot as easily as it gets in.  You can remove the slot but it still grips more in 1 direction. 

The fully enclosed holes definitely are less prone to becoming undone.  Without the fully enclosed holes, the string had a path to get between the 2 panels, working its way out from the hole.  They're more practical with #18 mason line.  

Clothing farsteners need a bigger opening, but that's going to cause more chafing.

More layers would be very time consuming to put on.    Then there's the idea of a 2nd opening which forces the string to go in the grippier way.  

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Eventually made some PLA cord locks with this design for #18 mason line.  The expansion properties of PLA required a lot of fudging to get them to fit.  

Also made a PLA cord lock with slot openings for clothing.  A key improvement was making the button much taller.  

Hard materials like PLA ended up gripping larger string like clothing draw strings better.  Soft materials like TPU ended up gripping #18 mason line better.  The advantage of hard materials is a more assertive button connection.  The 2 pieces stay together on large string more than TPU.  It's become common practice to modify the opening size with the soldering iron to get it to precisely fit.

Narrower slots, longer slots, thicker stacks, & taller buttons have been big improvements.  Taller buttons seem to allow some variability in the tightness, based on how far apart the 2 pieces are.

The last round of death stars continued to loosen in the drier, so that design seems finished.