Please keep in mind that freezing may affect the texture of the rhubarb dump cake slightly, but it should still be delicious! Let the cake cool completely before freezing. Cut it into individual portions or leave it whole if you prefer. Wrap each portion tightly with plastic wrap and then with aluminum foil to prevent freezer burn. Label each packet with the date and contents, so you know what it is and when you made it. Place the wrapped portions in a single layer in an airtight container or freezer-safe ziplock bag. Freeze for up to 3 months. When ready to serve again, take out the frozen packets from the freezer and let them thaw in the refrigerator overnight or at room temperature for a few hours before reheating as desired.