If *I* needed a temporary 120V feed, I'd get a 240V (dryer) cord, run
it into a outlet box, cap one of the hot feeds, and wire up the second
hot/neutral to an outlet.

If you're going to be opening up boxes, why not just move
one of the two hots to the nuetral-bus in the service panel,
tag it at both ends, and replace the receptical and breaker?