I had someone give me a key that was exactly the same as my apartment key. I designed this and the Autobot one as a means to differentiate them. This fits one of those hexagon head type keys. I printed this at 0.2 mm layer height and 10% infill. Then you will need to superglue the two parts together before inserting the key into it. The hardest part about this design is being able to pry the key ring open enough to get it on.