I was going to post this as a project but didn't get around to it yet. This assembly just sits on the reefer roof vent and the vent cover screws down over it. It's made with three 12-volt computer fans and a polystyrene cutting board. The fans are 120mm. You can size the amp rating of the fans as you desire, keeping in mind how much battery drain you want to tolerate. Run the wires down to the circuit board.

Add one of these to turn the fans on and off according to the temps that you set

I didn't take any scientific measurements but can tell you that my reefer now passes the ice cream test with flying colors. My feeble hands were overcome and I had to ask my wife to scoop it out for me.