Part: carabiner — a 60 x 34 x 6 mm ring, 5 mm band. A 28 mm opening in one long side is closed by mechanisms.leaf_spring: a 1.2 mm gate rooted in the ring at one end of the opening, bowed 1.5 mm outward, its free tip resting in a mouth at the other end under a 1.5 mm lip. Push the gate inward with a thumb and the tip swings out from under the lip, opening the ring for a key ring, a strap or a bottle loop; let go and it springs back. Anything pulling outward on the gate presses the tip harder into the lip, so it cannot open by accident.

Printed flat on the bed. The gate and the ring are one solid (the root is buried 2.5 mm into the ring); the tip clears the mouth by 0.4 mm along the gate and 0.35 mm under the lip, so it prints free without support. Gate root strain at 3 mm tip deflection is about 0.6%: fine in PETG, and PLA survives it for casual use (keys), not for hundreds of cycles a day.

Not load-bearing: keys, a water bottle, a lanyard. Never for climbing or anything holding a person.

Assumptions (user unavailable): ring proportions chosen for a thumb on the gate and two fingers through the ring; 6 mm depth so the gate is a 1.2 x 6 beam; no keyring hole (the ring is the loop).