This key was designed by The Open Organization of Lockpickers to open the widest variety of handcuffs today. It will work with dozens of models across the world, including some high security cuffs.
Start with an unpainted screw with a 3.5 mm shank and a Torx or similar drive head. Phillips or slotted will give you trouble.
Put a drop of oil in the head, and drill into it about 10 mm with a 7/64 drill bit.
Heat the head to red hot with a torch, stick the back end of the drill bit into the hole, and hammer it flat. Remove the bit. You might need to heat the head again to get the bit out.
Sand, grind, or file the head down to the level of the hole. Add another drop of oil, and drill into it again. Gently wiggle the bit around and keep checking with a caliper until it’s 2.9 mm in diameter.
Shape the rest of the head according to the above dimensions. I removed the bulk of the metal with a belt sander, and switched to files as I approached the final size.
Front end of the key finished.
Shape the back of the key according to your application. I heated mine, hammered it flat, and drilled a hole.