This is a little structure which lets you put a padlock on a PET bottle piece (in Germany known as a "PETling").
See attached photos about how to use this thing.
All you need to buy extra is a padlock where the clip has a maximum diameter of 7mm.
I'll also upload the scad file which I wrote to create this thing, although it was written quick & dirty and could still be optimized.
Originally the PET lock was intended as a wedding gift where the PETling hold some money and the newlyweds had to capture a travelbug in the wild to get to the key of the lock. :)
If there are any questions left, don't hesitate to contact me.