Buckwheat:

While you CAN put a fridge thermostat into a freezer the difficulty arises in maintaining that 80% humidity level.

You see, in a regular fridge, the humidity of the air is extremely low because the air inside the fridge regularily passes over the evaporator coils, which in a healthy fridge are very very cold. So, the air passing over a -60 deg. F evaporator coil is going to leave with a humidity of 100% humidity at -60 degrees F. That humidity is much lower than 100% humidity at 34 to 38 degrees F. So, almost all of the humidity in that air forms frost on those evaporator coils, and it's that removal of humidity from the air that allows the evaporator coils to go without having to be defrosted but only once every day or there abouts.

If you're going to be putting open containers of water in your upright freezer, then your evaporator coils are going to be continually caking up with frost, and you'll be continuously refilling those open containers with water. This is problematic because the surface area of an evaporator coil caked up with frost is far smaller than the surface area of the evaporator coil fins, and the result is that the fridge will warm up for lack of sufficient heat transfer from the freezer air into the refrigerant.

I think what you should do is Google "Cheese Making" and see what other people have done to create the right conditions for the bacteria to thrive.

I see your plan of using an upright freezer to provide the right conditions for bacterial growth as fatally flawed. The very way a refrigeration cycle works causes it to remove humidity from the air, and that's the downfall.