-20 C is ~ -4F. Key questions that needs to be answered before solutions can be offered are 1.) time spend inside the freezer and 2.) the actual temperature of the freezer or the rated capacity of the freezer. I have more than once been told the lifts are operating in -30F temps when the actual temp inside was -10F or even 0F. The freezers name plate had a capacity of lowering temps to -30F which to means to me a blast or flash freeze environment- like for ice cream makers. Guess it was still too cold to go inside & read the thermomter (just a bit of humor). Any operating temperature held below 32 F can be classified a freezer. But it does make a big differnce in how to best protect the equipment & allow full function for a "reasonable" operating time.