Question

design a device to prevent loss of heat from liquid other than a thermos flask

Answer 1

any container which can prevent the contact of the surface with the outside air will do the job, the best way to ensure this will be to make a hollow space int the surfac of the container by creating vaccuum but since heat travels in the form of EMR there will still b a small amount of heat loss,
to prevent this heat loss the best way b to use the minerals which have the property to absorb and retain heat, this will do the trick and absob all the heat from the liquid and use it to keep the liquid hot, but extraction of this minerals is very costly.

Answer 2

vaccum flask
a container that keeps the substances hot or cold by means of a double wall enclosing a vaccum