Letterboxing 4x3 media within 16x9 puzzle piece

I'm doing 3D twitch broadcast, a very experimental thing, and it relies on exact Left Right parity to convey the 3D images correctly.

I noticed when I switch my media from 16x9 to 4x3, the problem with your default method is that it grows the puzzle piece in height below where I place the picture so that I get an incomplete picture or it's covering something else I need.

I would like a method where I could take the exact same frame I'm meticulously measured to the nearest pixel keep that reserved as a 16x9 frame and then within that frame make a 4x3 picture that uses letter boxing to turn 16x9 in the 4x3 as opposed to stretching or zooming or growing the puzzle piece.

I know the video is 2D but the 3D effects in my room look good but the 2D image will not look right if I have to constantly change sizes and play with the positioning and sizing of my game window.

There should be an option that says for games that are broadcasting 4x3 like apparently my Lenovo monitor can do when it senses a 4x3 media it just put black bars on the side.

I want some method that will letterbox the video in a 16x9 frame with the black bars on the side so that it'll fit in the same puzzle piece when it senses 4x3 media playing.

If you already do that, let me know how to do that.

If you don't currently, please add that in your next update.

Manually calculating and placing 4x3 and 16x9 each time is very time consuming when I just wanted to fit inside the same 16x9 window when I play 4x3 media.
 
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