Put the sample in a walk-in refrigerator (with an electric receptacle or run an extension cord through a porthole/door), add a stir bar to your sample and place on top of a stir plate. Before use, equilibrate the stir plate to 4C (let it sit in the walk-in for a few hours).
I am in favor of using a small constant temperature bath, as also suggested by Ya-nan Wang. A big room or even a large refrigerator may not be a wise idea to use with 100ml of solution - it is a waste of electrical energy.