Looks like your moulding is also out of square since the top right miter joint has a gap in it. Take your time, do it right so it looks right when you are finished. I would pop the top horizontal and right vertical moldings off. Then I would square the upper jamb so that your opening is level, then reapply the moldings. The moldings will probably fit better and look much better to the eye. Then you can build two small, hinged doors, either flat panel or raised panel to match the closet doors below. This really won't take much time and will look so much better than a quick fix. You want to be proud of your work when you are finished, not wishing you had done a better job in the first place.