<p>Small urban waterbodies (SUWs) play an important role in managing stormwater, treating wastewater, and building climate resiliency. Recently, SUWs have been identified as potential sources of greenhouse gases (GHG) such as carbon dioxide (CO₂) and methane (CH₄), although quantification methods vary. To reduce contamination during sampling and improve spatial variance testing at a lower cost, a static, closed flux chamber with real-time sensors was proposed as an alternative to traditional syringe sampling and GHG analyzers. A floating flux chamber was designed and constructed using 3D-printed components and hardware-store items for ~ $820 USD. Validation experiments showed that real-time sensors reported 3.5% (<i>R</i>² = 0.9966) and 3.1% (<i>R</i>² = 0.9987) more than the values measured using gas chromatography (GC) for CO₂ and CH₄, respectively. The system was deployed in a wastewater lagoon in Almonte, Ontario, Canada, to provide proof of concept and calculate flux in a real SUW. Large increases in concentration upon lid closure were noted before equilibrium was reached between the chamber and water. Exponential and linear fits were applied to the recorded concentrations, giving fluxes of 892 ± 61 mg m⁻² min⁻¹ and 546 ± 37 mg m⁻² min⁻¹ for CO₂, and 254 ± 12 mg m⁻² min⁻¹ and 245 ± 11 mg m⁻² min⁻¹ for CH₄. The system is suitable for a wide range of fluxes across all types of SUWs, advancing research on how waterbodies factor into the global climate budget.</p> Graphical abstract <p></p>