Summary

This thing was made with Tinkercad. Edit it online https://www.tinkercad.com/things/joV10BvGTG9

I replaced the standard loop with a carabiner so that you could affix it to your belt loop.

Instructions

Check your sizing. I believe that the "F" shaped portion of the key should be .4 cm wide.
As per original instructions:
Again: it's important that you print this as sturdy as possible, to prevent it breaking off inside a handcuff.

Print it lying down. Use support material.

Make it as solid as you can. You can even play around with your slicer's fudge setting to make it more densely packed (in Cura this means lowering the packing density, in other slicers try the filament diameter). Using small layer heights (e.g. 0.1mm) also helps to make it more sturdy.

So, to recap:

Support material
Low layers
High fill
Really high fill
Print laying down

Handcuff key for belt loop