Place cake tier (uncovered) and the plastic plate it sits on into the freezer for 1 hour to let the icing firm up.
Wrap the frozen cake and its cardboard base in several layers of plastic wrap, making sure that air will be sealed out. You can also place the wrapped case into a lidded container for extra protection.
For best results, the cake should be frozen in a manual- defrost freezer. Self-defrosting freezers cycle on and off, and this can affect the quality of the case over time.