Lots of great binder models online, but I wanted one with:

• No magnets
• Using soft TPU as a printable insert to press against your wrists

The only thing you need is 10x 20mm m3 screws
(you could get away with less, hypothetically 2 plus glue)

WARNING: these cuffs should open when pulled, but, y'know, I'm not a toy manufacturer - just a dude on the internet. Your prints will differ slightly compared to mine, be careful and don't
To close the cuffs, simply press them shut and they will ratchet a bit.
To open them, simply flex the cuff-door inwards a bit, and pull out. The teeth will disengage and slide out.
In an emergency, you should be able to pull hard, and the soft teeth will bend, letting you pull the cuff free

For the tpu parts, I recommend a thin walls (2 perimeters), very low infill (~10%ish) and a soft tpu (85 shore hardness)

The design is parametric, so you can customize your own - if you have bigger wrists, want thicker cuffs, etc:
OnShape files here