Thingiverse user Thing-o-Fun from Austin, TX, designed this printable, fully functional padlock.
(A)ll items are printable except for the springs which I removed from ball-point click pens (thankfully my wife is still putting up with disassembled pens around the house). This one is a little more complicated but it still works and makes a good desk oddity. If you were to glue the “Top”, “Retainer”, and “Lid” in place, the only way to get this lock open without the key would be to break it. But, what is the fun if you can’t take it apart?
The above photo is actually a print of Thing-o-Fun’s design by Jon Hodgins.