If you want a classic, width adjustable border at export, I can highly recommend LR/Mogrify: https://www.photographers-toolbox.com/products/lrmogrify2.php
It adds a whole set of post-process actions you can use at export:
Select the “outer borders” action, and click “insert” and then you will have this options at the end of the Export options:
Works like a charm!
I’ll have to think about classic framing… it does feel a bit like cheating, and would be hard to make it not feel fake, but let me think about it…
-Nate