You could feed dry ice chunks into alcohol. This way you will have dry ice will induce a sublimation in alcohol bath. This bath is a great domain to generate a constant temperature bath. Sublimation temperature of dry ice is about -78.5 C (atmospheric pressure). I would pass dry air through cooling coil that is submersed in the alcohol bath in which dry ice is changing phase from dry ice to gaseous CO2. We would also mix this cold air with ambient air to control the final temperature needed in the experiment. Good luck and let me know how it goes with the experiment...
if you need much colder temperatures you use a liquid N2 bath with an immersed cooling coil... (around 77 K at atmospheric pressure)