Adding black bars the correct and easy way

I'm working on Premiere Pro CC 2018 and I had 4k and 2k mov and mp4 video footage mixed in 4 k timeline and since I have decided to export it in future in 2k (actually 1980 X 1080) so I recently changed the sequence settings from 4k (4096 X 2160 )  to 2k (actually to 1920 x1080).  I was told that I don't have to worry about downgrading the resolution from 4k to 2k but the timeline does it automatically, or I just hit scale or set to frame I assume to fit the 4k footage in the 2k timeline. Because of the 4k 4096 X 2160 aspect ratio and using warp stabilizer sometimes the 4k footage with its different aspect ratio in 2k(1920 x 1080) timeline creates a black jumpy bar on top and bottom but it doesn't worry me because I want to add a relatively thin black bar on top and bottom of the footage anyway to make the video look more cinematic. I watched tutorials on the correct way to add the black bars on top and bottom of the footage. There seems to be different opinions about it and since I believe there is a lot of updated knowledge here I wanted to find the easiest and fastest way to do this but also that it works after exporting and for the mainstream film buyers/distributors around the world.  Some tutorials mentioned the fast adjustment layer and cropping process but some said that is not a good way to go because it does not necessarily work with different tvs/aspect ratios etc.  Another option I noticed is somebody creating a title or color matte rectangular and adding it to the timeline as a better option (  i assume currently it could be done through the essential graphics somehow ) ?  Thank you in advance.