Using heat pack in 7x7x7 box's

Hi, I will be shipping a baby corn snake soon (when the weather where he is going is at least 40) and plan to ship him in a 7x7x7 box. Is it ok to use a 40-hour heat pack in a box this small, or would this put the animal in danger of overheating?

Per SYR heat packs should not be used in any box smaller than the mentioned size.