This is pretty much how the Canadian Army has done it for years and there are issued thermos which are the same size and shape as the water canteen so it will fit in the TAC vest. These only have narrow openings and are great for soups etc. As a general rule remember that nothing will stay hot for very long unless it goes in very hot. Preheating makes the contents last a considerable amount longer but Stews and foods with less liquid content lose their heat very fast even in the best thermos. Normal temp water still may freeze if left out in the Alberta cold overnight in a thermos (had it happen to me in Wainwright) but if heated enough before it goes in it will remain drinkable for quite some time. Anything with a Glass liner would end up broken very fast.