The 0c issue can be handled by installing a ventless catalyst heater then using a high efficiency computer fan to send enough heat to the storage compartment to maintain temps above 0C. You absolutely need to clad the interior of the compartment with hardboard cell foam paneling to reduce heat loss. The heaters range in size the 3,000 BTU is plenty. The heater can be adjusted down to 33% of 3,000 BTU will be a PITA to foute Use a hood above the heater to capture by convention and a good fan like an EB Pabst 24 volt running on 12 volts will not consume an excess of battery. The 4" flexible ducting will be a challenge but if you are serious it is doable.
The surplus center of Nebraska has the correct fan. See if you local heating contractor can sheet metal a stainless range hood to go atop the Olympian heater.