Fresh water bottle (do not burn through the polar envelope), which retains heat long! Up to 7 hours if well wrapped. Once cold, it is not unpleasant because the polar envelope prevents feel cold. Sounds like a simple cover.
By cons I do not think it makes 2 liters. Maybe if we filled background ... but it seems to me that rather puts 80cl of hot water in it. Finally it is more than enough for the desired effect.
