This is a great design, but it had really flimsy attachment points for Koss headphone elements. I broke at least five in the process of building a set for myself and a friend, printed in PLA+ and PETG, tried different print settings but there just was too little material for enough strength for the part to begin with.
I present to you the CHONKY CLIP Remix. As the mount point for headphone element is way more rigid, use hot air gun with small nozzle or jet torch with short bursts until you see first signs of PETG melting, then insert the element. Don't push too hard, you will break your element with excessive force.
With PLA you probably are fine with dip to boiling water for couple seconds.
The author remixed this model.