Just a cautionary note on thermos flasks if you use one of those with a plunger on the top.
I used to use these in my air taxi days and, if you do not add a little COLD water before t.o., the water reboils at altitude due to the drop in pressure, increasing the pressure inside the flask. When you then go to use it, the pressure inside the flask then keeps the (boiling hot) water flowing - can be very dangerous.