Crop a video to 2:1, the Univisium frame. A 1920x1080 source exports at 1920x960.
A compromise ratio, on purpose
2:1 was proposed by cinematographer Vittorio Storaro as a single ratio that could serve both cinema and television without either having to letterbox or crop the other. A lot of streaming drama is finished at it for the same reason.
It is wider than 16:9 and shallower than CinemaScope. Cropping 1920x1080 to 2:1 costs 120 px of height, against 262 px at 2.35:1, so compositions usually survive it.
Clean numbers
2:1 divides evenly at every standard size: 1920x960, 3840x1920, 1280x640. That matters more than it sounds, because H.264 encodes even dimensions only and ratios like 21:9 and 1.91:1 have to be rounded to the nearest even height before they can be encoded at all.
Keeping the whole frame
Fit with bars places the full 1920x1080 picture 1707x960 in the centre of a 1920x960 frame, with 107 px of background at each side. At 2:1 the bars land at the sides rather than the top and bottom, because the source is taller than the target relative to its width.