Eventually you’ll find yourself in a climbing situation where you might need a secure carabiner setup, but you don’t have any more locking carabiner nor an extra regular carabiner to rig the well-known “opposite and opposed” set up.
Provided you don’t need to open and close the carabiner, a simple solution might be called the “cheapskate locker”. Just use a regular carabiner and tape the gate closed, with whatever tape is (hopefully) handy.
Situations where this might be useful are setting up top ropes that will be in use most of the day (remember to always have at least two anchor carabiners for a toprope) or setting up a rappel where you are rapping off a single “leaver” carabiner.
If you plan on removing the tape, fold the tape and back over itself to leave a little tab for you to grab.
The “cheapskate locker” is is a favorite trick of big wall climbers, who always seem to be short on locking carabiners.