The use of solar passive hexagonal open-top chambers (POTCs) is a viable method for experimentally manipulating daytime air temperatures in low-stature plant communities at high latitudes. Here we describe a new hexagon POTC-based system that uses thermal inertia to increase overnight temperatures and variable chamber height to reduce overheating in summer. Field data collected in tall temperate grasslands show that the presence of thermal mass raised minimum and mean nighttime air temperatures by up to 1.5 °C while lowering chamber height, along with thermal mass, limited the development of extreme daytime chamber temperatures in summer. We also demonstrate that, by using a simple, inexpensive twin carbon dioxide (CO2) injection system regulated by an infrared gas monitor, it is possible to generate targeted and stable atmospheric CO2 enrichment within these chambers. These innovations significantly improve the conventional hexagon POTC design and represent a low-cost method for assessing the effects of warming and CO2 enrichment on low-stature vegetation in low latitude environments.