Cocktail Cabinet mirrored shader problems

[Puchi25]

I've recently have gone through the steps of installing a glsl shader to Retropie in hopes of achieving a split screen mirrored image for two player games on a cocktail cabinet set up.

(The shader I used: https://github.com/libretro/glsl-shaders/blob/master/misc/cocktail-cab-portrait.glsl)

I turned on the shader setting in retropie set up, then started messing around in the retroarch game menu where you can apply the shader. I eventually did and became very excited when I saw the a mirrored look. From reading through forums I knew the next step was to adjust the parameters setting to get a preferred look. However, I instantly noticed how limited my parameter space was. It seemed as if the mirrored shader had worked, but had not unitized the full length of the monitor. It worked in the default space it was provided in.

Here a picture to help my explanation:

The shader is working but only in minniamal space that I can't seem to change.

In the image below, I adjusted the parameters of the game to how it should be fitted in proportion to the monitor.

As you can see, black bars are covering the tops and bottoms of the screen, where the game otherwise could have been seen. How can I adjust these bars?

I've tested multiple shaders and can't seem to find a solution. I also looked around in the retroarch video settings for possible adjustments. I couldn't find anything. Any help would be greatly appreciated. Thanks!