Every Excel version since (I think...) 2003 has defined a very well thought-out XML format, which is easy to build using a template.

AFAIK, if a file with the extension “.XLS” actually turns out to contain XML content ... “it just works.”

I have used a similar strategy to populate a (live!) Excel spreadsheet within a generated web-page.   As long as the MIME-type is correct, it works.