Designing large-scale fog computing platforms requires the aggregation of large numbers of servers in numerous locations covering a full country or even a continent. However, it would be difficult for a single organization to deploy enough resources while attracting sufficient workloads to generate high resource utilization. Instead, we propose the concept of meta-federations, where many independent local resource providers may lease their resources to multiple fog federations. We propose UnBound, a scalable meta-federations framework that specifically addresses the difficult multi-tenancy issue in this context to isolate workloads belonging to different users and fog providers. Extensive evaluations with federations of up to 500 geo-distributed Kubernetes clusters demonstrate that UnBound maintains comparable application deployment times to the state of the art in a single member cluster scenario, avoids increasing cross-cluster network traffic, keeps resource consumption within acceptable limits, and exhibits stability and scalability, making it a suitable solution for large-scale fog computing deployments.