[Sunny Sawrav]

Why don’t you try sending the project to Compressor? 🙂
After you’ve edited your project and are ready to render it out, go to File -> Send To -> Compressor

There, select a setting that best suits your render output or right click on a setting to duplicate it and tweak it to suit your needs.

You can tweak the settings in the Inspector Window.

Coming back to letterbox, there’s a tab called ‘Filter’ in the Inspector Window. Click that tab and scroll through the given filters. One of them would be Letterbox. There are presets you can use or define a custom setting. 🙂

Cheers.