Hacked together a service in Go https://github.com/xlab/cdn-s3-go that routes content request to any arbitrary S3-compatible bucket.

Some infra providers are offering free egress, but buckets are only private (hello Railway), and not geo-distributed. An L4 balancer will cost a fortune (hello Cloudflare).

cdn-s3-go solves this by looking up objects, generating a pre-signed URLs and redirecting the clients directly. Free egress, cheap public geo-distributed CDN!