Text in Sidebars

[ Moved to the Fixing WordPress sub-forum. ]

On my site, which uses a custom WP theme, we wanted to place a small block of text that would accompany a post. For instance, a post might be about a new book, and the text snippet in the sidebar contains basic details about the book. See: http://www.williamstewart.me/a-fortunate-land-the-norse-in-greenland/

To implement this, we used the “Excerpt” service, and so the sidebar displays the Excerpt for the given post that is being viewed.

I’m now working on a site with the Twenty Seventeen theme, and was wondering how to do this with a generic Theme. It seems like a useful feature to me, to have a snippet of text associated with just that post, appear on the same page, in the sidebar or footer, of just that post.

So, I thought there might be a widget to do this, but now discovering that Excerpts aren’t really meant to be used this way!

I’m really looking for a plugin, or some way to customise a standard theme, but thought it might be interesting to post here, as this might be a feature worth considering?

Thanks

William