How can I create a virtually lower and letterboxed resolution?

I have a screen which only accepts the 1920x1080 mode (others are garbled and adding custom modes does not work either). However, the screen has a centered viewable area of 1440x1080. That means the external screen crops the left and right.

I would like to make the resolution of that screen virtually smaller while keeping the same display mode, effectively adding letterboxes on the left and right. I have been trying some things with xrandr but I always ended up scaling the output which is not what I want.

Example for the result I would like to achieve

Thanks a lot in advance!

Edit: I'm using Ubuntu 21.04

tl;dr: I would like to create a virtual screen of 1440x1080 and display it on an external display which is fixed to mode 1920x1080.

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