Photo Letterboxer is basically a universal filter. It shows up in your sharing menu as a share target; you can share any JPEG or PNG to Photo Letterboxer, pick the proportions you want, and the color of the border, and press the Letterbox button. You'll be given another share menu; choose an app, and that app will start up and be given the letterboxed image. (If you want to save it to a file, I recommend my Save To File app.)
Sharing, uploading, downloading, just to save to my camera roll is a lot of steps. Needs square option and custom size option.