I used an old US Article flask. Doesn't look truly authentic but its twin wall insulated and is, in effect, a Thermos. Ice cold lemonade on a hot day from my flask makes a nice change
Flask 2 by British Red, on Flickr
I did try it and seem to recall it was fine for a few hours. It was a long time ago though.
http://www.ebay.co.uk/itm/GENUINE-U...-amp-CUP-/311270724607?_trksid=p2054897.l4275
I think the design was to stop water freezing rather than using it as a 'Thermos Flask'...a bugger to clean if you filled it with hot chocolate lol.