Efficient secondary use of real-world data (RWD) is a cornerstone for advancing data-driven medical research and personalised healthcare. However, significant challenges persist, including data fragmentation in silos, the lack of record linkage, and legal constraints that often hinder data utilisation. Especially Electronic Health Records (EHRs) represent a valuable data source, yet their potential remains largely untapped due to these barriers. Especially modern data space solutions promise to address these challenges, focusing on standardisation and harmonisation efforts, data governance aspects, as well as federated data-sharing approaches. A significant push in this area represents the European Health Data Space (EHDS) Act, focusing on an opt-out based approach for secondary use of health data. An additional consent-based approach (opt-in) represents data donation, which empowers individuals to contribute their data to research while maintaining trust and privacy under the current legal situation. The flagship project Smart FOX lays the foundations for making citizen-based data donations of EHR-standardised information usable in clinical research in Austria. As part of the architecture of the Austrian Health Data Donation Space (AHDDS), data donation boxes - Federated Open data eXchange Boxes (FOX BOXes) - present the fundamental decentralised building blocks for sharing EHR-standardised data. This paper outlines the architecture, functionality, and governance of FOX BOXes, highlighting its role in overcoming key barriers to health data sharing and its potential to accelerate data-driven research.