Various plasmonic nanocavities possessing an extremely small mode volume have been developed and applied successfully in the study of strong light-matter coupling. Driven by the desire of constructing quantum networks and other functional quantum devices, a growing trend of strong coupling research is to explore the possibility of fabricating simple strong coupling nanosystems as the building blocks to construct complex systems or devices. Herein, we investigate such a nanocube-exciton building block (i.e. AuNC@J-agg), which is fabricated by coating Au nanocubes with excitonic J-aggregate molecules. The extinction spectra of AuNC@J-agg assembly, as well as the dark field scattering spectra of the individual nanocube-exciton, exhibit Rabi splitting of 100–140 meV, which signifies strong plasmon–exciton coupling. We further demonstrate the feasibility of constructing a more complex system of AuNC@J-agg on Au film, which achieves a much stronger coupling, with Rabi splitting of 377 meV. This work provides a practical pathway of building complex systems from building blocks, which are simple strong coupling systems, which lays the foundation for exploring further fundamental studies or inventing novel quantum devices.