Electrical Questions? Ask an Electrician Online.
Welcome. My name is ***** ***** would be glad to assist.
Is the question about having the 6" opening and only being able to use a 4" n=box, leaving the space around the box open?
Or, is it something different?
Let me know and we can continue,
You would use an old work bar hanger. It fits between the joists and then you tighten the bar through the opening and it snugs up between them.
It is rated for ceilings fans also as well as fixtures.
CLICK HERE to see one
There is a video on that page as well to view installation I believe
You are most welcome, glad to assist.