The capsule needs to be printed in TPU, as it relies entirely on friction.

If the fit is too loose or too tight, try scaling it up or down in your slicer.

There are two versions of the key available.

The hole for the string in the capsule is 5mm in diameter.

I don’t have any handcuffs to test this on (But I have an old key), so feel free to leave a comment with your feedback. You may need to drill out the hole in the key, as making it larger in the model would not be very print-friendly.