I mocked this up to use as an icon for the Journal app, but it could work as a general journal/diary icon.
I took the book icon and added the lock from the folder-lock to the book icon. I'm working with shitty web image editors, but if somebody could polish it up I think it'd make a good addition to the library.

Plop the SVG in a zip here.
Unfortunately, I don't have the .svg or any way to even make one. I only own Chrome OS devices with no real photo editing software. I use websites like https://www.photopea.com/ and https://pixlr.com/editor/ for all my editing needs.
We should call it book-secure to match the other books with an alias of journal and diary.

Do you think that lock looks better than the one used on the folder-lock and file-lock? I like the way the curved edges match up on yours, but I think the squarer icon works a little better at the small size needed to fit _inside_ the book icon.
Thanks, though. Whichever you prefer!
You're probably right, with the folder-lock icon, we can have an unsecure version as well.

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You're probably right, with the
folder-lockicon, we can have an unsecure version as well.book-secure.zip