This paper presents a testbed for the ECHONET Lite Web API (ELWA), a protocol supporting cloud-to-cloud interoperability for IoT systems. The key innovation of the testbed lies in its ability to integrate IoT devices using any protocol or communication pattern through the introduction of an MQTT-based protocol binding and a token-based access management mechanism. This architecture enables a two-sided marketplace model, with the testbed server acting as a mediator between IoT device providers and service developers, realizing the testbed-as-a-service paradigm for the ELWA ecosystem. The proposed system has been implemented and deployed in a real-world environment with a variety of devices. Performance evaluations demonstrate the testbed’s scalability, reliably handling up to 3,000 concurrent requests per second, with minimal overhead introduced by protocol binding and access control mechanisms.