Thermo bottle for impromptu sous vide?
I don't think a thermos bottle will give you enough room to cook much of anything.  However in principle, it could work.  It would be better to use a cooler.  This old Serious Eats article explains everything.  Of course, that was written 8 or 9 years ago, when immersion circulators were very pricey.  You can get them now for as low as $99 US.

You won't need 12 hours (and I wouldn't recommend any long duration cooks without an actual circulator).  You can easily cook a steak (even in a pot in the kitchen sink with running hot water if your water heater puts out high enough heat) in an hour.  You could also do this on the stove top, controlling the burner and monitoring the water temperature, but a cooler will retain the heat and you will be pretty good for a cook of an hour or so.

I've been using sous vide for at least 10 years.  I don't use it for everything.  It's one of a set of kitchen tools.  However, I do find it useful.  Given the current price point, I would say it is worth the purchase.  Plus, then you have the most control, with less monitoring...plus the ability to experiment with much longer cooking times (which impacts texture of proteins).