The Bachmann Knot is an old knot that has stood the test of time. I've shown the knot with two turns, but one or two more can be added to increase the holding power. This can even be done in the middle of a climb, if needed. Placing the carabiner gate so that the opening is on the bottom helps keep the upper bight from accidentally coming out of the carabiner, which results in an untied knot and a dropped carabiner. If the lower sling comes out, all that is lost is one turn in the knot. Once climbing has begun, the orientation of the carabiner on the standing line does not seem to matter much, and many people don't worry about how it sits.
The carabiner is not a handle, and the knot will jam if the carabiner is raised and slip if it is pulled down.
The Bachmann knot will hold if nylon webbing or Spectra slings are used instead of accessory cord slings, but you may wish to add an extra turn or two. It does not hold well if inverted (and it can be inverted without removing it from the rope), and it tends to slip if it is not loaded.