I used the handcuff ratchet system as a clamp for another project but then went on to re-purpose the design into this toy handcuff design. That’s why its keyless and better as a toy.  I’m not sure they could restrain anyone for long, which is probably a good thing. Even though they are keyless, there is an alternative shorter lever STL that is recessed and needs something small (the key) poked in to release the mechanism. So you make a keyed version. This is fully printable, so you don’t need to find any additional hardware like screws or use glue. You can print all the parts in one go with a suitable sized print bed, but I recommend printing the chains separately using a brim. The chains also print more reliably using a low resolution 0.2 - 0.3mm.