I use lab chiller that goes down to -18 C with 60 plate heat exchanger. Then with insulated silicon hoses I can create large separate water bath that can be as cold as water can get, in insulated bucket that is the bath. The chiller circulates coolant that cools the water part of the system through the heat exchanger. Care must be taken the water does not freeze inside the exchanger. Also reusable freeze packs or whatever they are called (0.5L plastic containers) be worth putted in ur bath to help process.