Yes, you can get plastic wide necked flasks. Soup is a good idea, but don't make it too hot - I know that sounds mad, but DD1 only gets 20 mins to eat her dinner and she will huff at hot soup for ages and would get two sips out of it before the bell went!
Home made soup is a very good way of smuggling veg into the diet too. DD2 claims to hate carrots and cabbage, but will eat "ham and lentil" soup with these hidden in the soup and thoroughly liquidised.