EDIT01: Now with a bunch of double variants!

I forget where I saw it, but someone made a nice one-piece carabiner, made from solid titanium. I've yet to make this from such a fancy material, but it's one of my favorite test pieces at the office and not too bad for light-duty fastening.

Up

Up Plus

No

20% at least

If you're printing on a flat bed like glass, you'll probably need to trim the gap between the body and the spring arm. On our current printer, it tends to widen on the first layer, which in this case would cause the active part to stick to the static perimeter. Of course, printing on a raft would prevent this.