I saw a stainless flask fall out of a moped riders haversack, bounce along the road a few times before being squashed flat by an artic and that's probably the only way a stainless flask could be damaged.
I also think that stainless flasks are the way to go for the OP. the two I have keep water scalding hot for at least 12 hours, minimum, and I always pre heat the flask with boiling water before filling, even if the final liquid to go in the flask is boiling water.
matalan and wilko are just some of the stores which seem to have inexpensive stainless flasks in stock, summer or winter and a (colective noun) of flasks is far, far cheaper than his only other viable option: a trailer containing a 3Kw genny a domestic kettle and one of these
RT