Thanks so much for the info and tips. I am really impressed with the quick replies!
I have re-examined the door jam, and found that it has a large and firm enough area to add a tension bar and small curtain (less than 6"). A curtain won't be sound-proof (but neither is the door!) and it will still allow for air-flow.
Since it was asked, we want to close the gap because the mirror and some interior of the bathroom (including shower!) can be seen from the top bunk.....!!
I will try to post pictures of it when done... heading off on another adventure, and I have lots to do, including hanging curtains and hardware for all the other windows. I've been trying to not physically alter anything or leave marks as I want to keep it in a decent for resale. Maybe the next buyer will appreciate the changes, maybe not.
Thank you!!