Refrigeration for Cryochemistry −35 °C I need to cool a tube containing liquid to −35 °C for a cold chemical experiment that will last about a week. The glass vessel I need cooled is 500 mL in volume and will be giving off $\ce{H2S}$ gas. I currently only have a fume hood. Most freezers do not seem capable of this low temperature. Most cryochemistry refrigeration seems designed for much lower temperatures, and is needlessly expensive. I realize that refrigeration is relatively simple: 1 compressor, 1 expansion valve, two heat exchangers, and electronic control/monitoring. My first impulse is to modify a freezer, but I am completely new to this. At this point I would be very happy with some general advice and perhaps a reference for custom designing these sorts of cooling devices.
I would suggest a Frigorific Mixture of calcium chloride (often sold as DampRid) and Ice in a cooler.  You will need quite a bit of each and to moniter at least twice a day, but it wont break the bank and can reach the desired temperature.