I also got a 500mmm one from Wilko - reduced to £3.00 (from £5) . Works a treat and keeps my cold drink nice & cold, even through the very hot days of last week. By comparison, my second, uninsulated , plastic bottle had water hot enough to make tea with.Mine came from Wilkinsons (Wilco) around £6 each. (350 and 500)
and I have one from LIDL about £4.
The former I often use for coffee and it stays hot for many hours.
Wouldnt you be better off putting a full flask of fluid in the fridge overnight (without the lid) as it will still chill down but wont risk issues with the ice lump?You don't need ice cubes or gadgets.
Just put an inch or two of water in a Chilly / Thermos and leave it without the stopper in the freezer overnight. Fill with drink in the morning. It'll then stay cold for far longer than is useful.
Wouldnt you be better off putting a full flask of fluid in the fridge overnight (without the lid) as it will still chill down but wont risk issues with the ice lump?