[BlackCaz]

Curious. I was under the impression that borders like the one you describe, BlackCaz, are actually shaders in their own right.

Actually, the example which I proposed is utilizing, both, a shader and an overlay. A shader directly post-processes the actual gameplay image which is being displayed, and the overlay is merely a border which outlines the gameplay image (in this case, a GBA console). They are separate, independent entities, in their own rights. In order to fit the gameplay image into the border, you must allow custom aspect-ratios. I suggest looking into RetroArch to use the existing ideas of how this can be accomplished. RetroArch also has the ability to adjust the overlay's opacity and scale it's size by zooming in or out to have it encompass the custom aspect ratios.