SQLar[1], see [2] for some reasoning around why a database is preferred over zipped XML. Though, I'd be fine with a DBM-style database too as we only need the key-value part of it.

[1] https://sqlite.org/sqlar/doc/trunk/README.md

[2] https://www.sqlite.org/affcase1.html

Compressed JSON with the binary content encoded in base64 strings, obviously.