Makerspaces have emerged as potential facilitators of the transition from the linear economic model to a circular economy (CE). This transition necessitates collaboration among diverse stakeholders to create sustainable, yet resilient systems. However, fostering effective collaboration faces numerous barriers, including misaligned interests, competition dynamics, limited knowledge experience, power asymmetry, and lack of trust. Our paper addresses these challenges and proposes an alternative tool for fostering collaboration: an online board game called Circular Synergy City. Unlike existing games, Circular Synergy City focuses on collaboration, allowing players to interact, negotiate, and plan with each other to explore potential synergies. We tested this game with 160 players in two play sessions. Based on the feedback gathered during these sessions, we present recommendations for developing collaboration-focused content, and board games.