Innovation in finance is crucial for developing countries to reduce poverty and achieve financial inclusion. By enabling interoperability of financial big data among all actors, Open Finance can promote innovation and competition. However, existing Open Finance models do not address the specific challenges of the African financial ecosystem, characterized by outdated IT infrastructure and high mobile channel usage. This study proposes a simple, secure, and cost-effective Open Finance solution that processes large volumes of financial data using open-source technologies like Keycloak as an identity provider. The architecture enables rapid deployment of Open Finance infrastructure in West African countries and is applicable to any developing nation. We implemented and tested the system by simulating an account aggregator accessing financial data from banks and mobile money operators. The results demonstrate secure sharing of user financial information and initiation of account-to-account transactions with user consent. The proposed architecture simplifies financial data sharing while ensuring data security and confidentiality, marking the first contribution to propose an Open Finance deployment infrastructure for developing countries.