The nieces have these insulated food jars they use for hot lunches.

But they say that by the time they get to lunch, the food's warm, not hot. No doubt warmer than it would be without the insulated jar, but....

I'm thinking that like a tea cozy would help keep them warm longer. What would be the best material to make such a thing out of? (Can't find anything on Etsy, and the company doesn't make any of its own.)