If you ever try to watch letterboxed video on a widescreen monitor you are given the worst experience possible: black bars on all four sides and a tiny video area.
The trick here is to stretch the video to fit the width of the screen, and allowing the top and bottom black-bars of the video to be cropped.
I couldn’t find an automatic method for achieving this, however the manual method is doable if you have a few minutes to spare. It goes as such (instructions are specific to ZoomPlayer but the method should work with any player):
Open the video in fullscreen as you would normally. Expand the video vertically until the sides of the video just touch the edges of your screen (Key: Alt-Up/Down arrows). Save this to a size preset (Ctr-0..9).
Now exit out of fullscreen and open the preset editing menu (Ctr-P). You should see the preset you just saved. Now you want to center the video vertically. Calculate the difference between the height of the preset video, and your monitor’s resolution – divide this number by two and put in the Y-Offset field as a negative value.
Return to fullscreen mode and apply the preset (0..9). Your video will now fill the full width of your screen and keep the correct aspect ratio.