We actually live in our basement, so that it is never going to be below the mid 60's. Our storage unit is in a complex so it is not a structure with 4 sides exposed, so it should stay above freezing.
I'm about 90 percent sure that it was a rat, because the hole chewed was just over fist sized. Next summer I will likely use your waterland tub/wire idea, as that seems more secure than the pen, which was frame and wire.

Thanks for all the great information!!!