I have a simple half litre steel vacuum flask from Ikea that works well with my my bottle cage. Filled with hot tea It will hold it's temperature quite some time. As soon as some liquid is out, the temperature drops faster, so for longer rides I prefer two 500ml flasks to a 1 litre one. I have a 350ml flask as well, but I find that one too small for most days.

If you have a pannier, backpack, or frame bag to put the flask in, rather than a cage, then the temperature will hold longer.