You said you need it to be 10° to 15°F above ambient, so you will always be heating, never cooling.[SUP](1)[/SUP]
But I will agree with Ron that, with one of the biggest side open, you will have a lot of air flowing in and out of the box. Remember, hot air rises. Does the side need to stay open? Can you have a door that closes to help contain the air?
Note 1: Unless there is something else in the box generating heat. If so, you need to know how much heat it generates.