You need to buy at least 1 Thermo flask for the Chadar Trek and other winter treks as one of the water bottles.
Recently i did the Chadar trek in 2018 and i wanted to ensure that i have warm water even in the minus 30c at night.
I tested 4 thermo flasks ( 3 well known brands and 1 not so famous) in mumbai at home.
I filled these thermo flasks with hot water and kept them in my refrigerator top section where we keep water to form ice.
I kept them overnight and in morning checked the water temperature in all of them.
Surprising only 2 of them had hot water and the other 2 had cold water.
The 2 which had hot water in spite of being kept in the freezer over-nite were Milton and Cello.
You can buy any of these 2 brands as i found that they are the best water bottles for drinking warm water in winter treks. These bottles have dual caps and you will never have issue of water leaking from these bottles.
In my opinion having a thermos as a water bottle is must in the cold weather conditions of the Chadar trek or any other winter trek.
Preferably buy the 750 ml as the 1 litle water is too bulky to carry in your back pack.
Caution: Never drink water straight from the thermos as the water may be too hot in the thermos and you may burn your tongue.