I found the right romex ends as well as the BX(ac90) ends, and ended up with a metal conduit box which had 10 different knockout locations on it, and screw holes in various locations making it very easy to attach inside a finished ceiling. Thanks for the help, was an interesting fit.
One question on the BX though - I had trouble shoving those plastic red anti shorts into the cable. It seemed to be very difficult to get over the paper inside the cable. It took me a lot of effort. Does it matter if the anti short goes inside or on top of the paper inside the cable and how far should they insert into the cable?