The original by Bababbak is a lovely design and does what it needs too with allot of practical simplicity. 
But the authors own recommendation was to print a remix with a metal spring. 
I've always been a bit of a purest in finding ways to think around the need for non printed parts. the original had printed flat springs which would put all of the flex in one small area leading to early fatigue. PLA springs can work just fine so long as the bend is small and spread over a wide area, i have redesigned the spring so that both half's are now the same piece (thus doubling the effective length of the spring) and set into a curved shape making the spring longer still and spreading area over which it bends. 

Other changes… 
Tapered the area where the cuff swings into the cuff so its less likely to jam. 
Tapered the hinge posts so that they seat easier as its a tight fit (you might want to add glue for strength) 
Modified the key so it no longer requires supports and down scaled it a tiny bit as it was a tight fit. 
lanyard hole to allow you to hang it or tie it to something… 

Prints settings 
Prints solid with 3-4 perimeters
The key will do fine on 2 perimeters and 30% infil 

Future development 
it could do with being able to lock one ratchet smaller than its minimum size 
The square corner invites warp and aught to have a slight radius