Single pretty url query var parameter

I’ve registered a custom query var that I read from shortcodes running on a page ‘d’, so at the moment … [URL]/d/?v=value works.

The page is essentially a template and the value is actually an author slug, but what I would like is to have a pretty URL like [URL]/d/value (I use the author base (author_posts_url) & a template for actual profile pages, this is a kind of white-labelled form page for each user and I would like to keep the URL as short as possible, also unlikely to ever need additional variables and so would like to not even have the key of the key-value pair in the URL).

I’ve tried experimenting with add_rewrite_tag and add_rewrite_rule with no success (possibly because I don’t know regex), and completely glazed over when looking at rewrite_add_endpoint.

Any help or pointers will be much appreciated.