Convert aspect ratio with pillarboxing (letterboxing)
I have a 4:3 video I need to display in Powerpoint which defaults to 16:9. Since most of the other videos in the presentation are already 16:9, I can't change the aspect ratio in Powerpoint or it all goes flooey. I can crop off the bottom or the top to fit 16:9 using iMovie but I lose that part of the video. So how can I convert the 4:3 to 16:9 with those pillarboxes (letterboxes) on either side so the video shows properly? Can I do this in iMovie or is there some other program I need to use - preferably free. Thanks for you help.
iMac 27″, macOS 12.2