I hate any bulk to my keys. So, I only carry around the keys that I need. This buckle acts as a quick disconnect for some of your keys. Maybe for 1 key or for a whole set. Colour code them even! These are both smaller and lighter than the metal disconnects that I found at the dollar store.

Taking the car? House and mail + car keys.

Taking your bike? House and mail + bike keys.

Going for a walk? Leave everything at home except just the house and maybe mail keys.

Carry a USB drive or other utility piece on your keys that makes sense to disconenct when using? Connect it too with a buckle!

Other buckles are just too large for keys. This was designed to be as small as possible without being too fragile or loose. Recommend printing out of Prusament PCCF for the male end and PETG for the female end. At 0.20mm layer height with a 0.4mm nozzle. 100% infill for strength. These parts can be very sensitive to changes in part orientation (ensure it's oriented like the “Key Buckle AB” file), and filament.

The female buckle prints better vertically on it's side but you may need a brim to get away with that depending on your bed adhesion. I didn't need a brim. No supports needed. 

Update 3: Tested the print orientation of the female end of the buckle.  The orientation of this part is important. Print the female end of the buckle on it's side like in the “Key Buckle AB” file.

Update 2: I found the male end of the buckle to break after about 3-4 months of daily use when printed with PETG. I reprinted in Prusament PCCF and that's much better. Prusament PC Blend should work too.

Update 1: I've added my Fusion 360 file, and a picture with the primary settings for adjusting the tolerances. Mainly, parameter “a” in my testing has the biggest impact on fit of the buckle. Smaller values will be tighter. Smaller values for “hook_radius" will be tighter as well. Larger values for “b” will be tighter. Change in 0.1mm steps one at a time. It took me about a dozen prints to get this to my default values that work for me.