What Is The Best Container To Keep Food Hot?

How do you keep food warm in a bento box? Highly recommend to re-heat pre-cooked/leftover food before packing. If you do so, I also recommend to heat up with a pan or toaster oven instead of microwave so that food, especially meat, is heated THOROUGHLY and not partially. Before you pack cold/cool food in your bento box, hot/warm rice must be cooled down.