I have been using
BorgBase to backup my Calibre library. I'm using the 10GB free plan and it's working perfectly.
They have a
desktop application that makes it easy to configure your backup, you can even set up a schedule to automatically update your backup and since it only works one way, you don't need to keep two copies of your library or worry about database corruption. It also creates incremental backups. I believe it is a solid option for free cloud backups.
Notice of Non-Affiliation
I am not affiliated, associated, authorized, endorsed by, or in any way officially connected with BorgBase, I just like their service.