In reply to Craig:
I have an el-cheap stainless flask I bought in Tescos.
if you pre-heat it it does keep my soup hot enough to drink most of the day as it is, in a foam caddy (made out of bits of foam mat, it is better.
I have noticed the stopper gets warm and the other day the plastic insert on top of the stopper fell out, and no wonder it gets warm, the stopper is hollow. I reckon filling the stopper with foam, ought to improve the performance of the flask quite a but.