Conventional design of DIY cloud chamber uses IPA and cooling down to -26..-33°C (sometimes even lower, down to dry ice temperature).
What could be changed in design approach to shift operating point closer to room temperature? -15°C is much easier to maintain with single-stage TEC compared to -26..-30°C. And something around 0°C could be easily achieved in large volumes. On the other hand - heating (if needed) is much easier than cooling.
For me it is not clear why optimal temperature is so low, and hence unclear whether I should pick liquids with lower or higher boiling point (acetone / water or other non-toxic solvents).
Answer
If you prefer to avoid low temperatures, maybe you should use the original approach to supersaturation - pressure change (see e.g. https://www.citycollegiate.com/wilson_cloud_chamber.htm).
EDIT (5/26/2019): The OP asked for a "steady-state" solution in their comment. Such a solution is described in R.S.I. (Review of Scientific Instruments), vol. 10, March 1939, p. 91, http://hep.ucsb.edu/people/hnn/cloud/articles/ALangsdorfRSI10_91_1939.pdf). It includes heaters, vaporizer, and a refrigerator, which can apparently operate at ice (not dry ice) temperature. See also the literature review there.
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