I work at https://kloudless.com https://kloudless.com. While not S3-compatible, we offer a similar proxy that provides a single API to multiple storage services

I work at https://kloudless.com https://kloudless.com. While not S3-compatible, we offer a similar proxy that provides a single API to multiple storage services such as Dropbox, Box, Google Drive, SharePoint, etc. We've also released support for S3 and Azure and are looking into B2.