Hi, We use igloo coolers and three ice blocks frozen and stored at -20 to -25'C. No colder. One refrigerated coolant pack (stored in a fridge of 1-6'C) placed on the bottom of the insert. Then the units on top of that and another refrigerated coolant pack on top of the units. Coolers will stay within 1-6'C for 8 hours. We can only send out two units in a cooler at a time.