I did a little digging and they don't open their API up so no other clients exist. I recommend getting some s3 compatable storage from any Cloud (linode, aws, etc) and using restic (https://restic.net) for your backups. If its more for syncing between devices I'd recommend syncthing (https://syncthing.net/) which is also opensource and will sync between devices without the need for a central server. Good Luck.