How to customize resolution without losing the proportion?

[IanTheWizard]

I'm playing in windowed mode in 1280x960 resolution. If I choose a resolution which is not the same proportion, graphics in the game would be out of proportion (such as being horizontally stretched).

How to make windowed mode wider and also keep the proportion?

There is a guy who provided a solution which requires installing a mod called Starter mod. His successful result can be seen here. But I'm not sure if the Starter mod also modifies the game content or mechanism. Has anyone here tried that mod?