Keep it between 15-30 celcius. To damage it at high temps it's gotta be long term and well over 30C (86F) consistently. Also keep it out of direct sunlight, so yeah a metal box is fine. It got super hot in my house over the summer and it did get up over 30 degrees, but not all day and at night the temp would dip a bit. I got a bit anxious about storage too. Now I just keep mine at my Dr.'s office, so that when I show up for my shot it's all good and air conditioned at room temps.