If you’re interested in using borders, I can’t help you – my suggestions are for taking maximum advantage of the display area and minimizing/eliminating letterboxing and overscan, so that borders aren’t necessary. Pillarboxing oth is a non-issue, as you get pillarboxing on a modern display anytime you view 4:3 content.
It wouldn’t be that hard to combine any of the scanline overlays with one of the console borders that comes with RA using a graphics editor.
You could just select the Hyllian shader and then add a border overlay. That sounds like what you are after.