Well, the problem is that if you put anything on the face, it will stick out past the doors which will interfere with fully opening the door. In the long run, there are numerous door options that have louvres that will satisfy the need for ventilation without looking like the folding doors you have. In the interim, maybe this would work - you can pull off the top doors and frame out the rectangle above the doors. Set the frame back 1/2” then put a piece of drywall up and paint it. That will make it look better until you can redo the whole door. If you are concerned about ventilation, a simple rectangle with a return air vent cover would look fine.