Links to custom html pages on a site with custom permalinks

I’m trying to migrate an old site to WordPress, and am having some trouble with a few old files who are so well indexed in search engines that I don’t want to change them.

The old structure:
https://www.domain.com
https://www.domain.com/page/index.html (default page)
https://www.domain.com/page/files/whatever.html
https://www.domain.com/page/files/whatever.other

I’ve gotten the new WordPress site working with:
https://www.domain.com
https://www.domain.com/page

I’m having trouble with the “whatever” URLs. I’ve tried creating a page called “files” with a parent of “page”, which works well enough. If I then create a page called “whatever.html” with a parent of “files”, it renames the URL to whatever-html and I can’t seem to get a “.” in there.

Suggestions are welcome and appreciated!

Thanks!