I can recommend using Minio - self-hosted S3-compatible storage server.
Minio is known to work well with CloudBerry Lab products if runs on a Unix-based OS, not Windows. Windows compativility issue will be fixed soon.
In case that's a Windows machine I can currently only recommend adding it as a network share via "local file system" account menu. Perhaps some other Openstack self-hosted solutions would also work fine for you.
that is why i put this in as a "Feature Request" the provided options are not viable for me at the moment. im currently using SMB so when you have a better option i would love to know about it.