I've never tried any of it personally but from what I've read about such gadgets is that they work maybe once or twice but
because it's electronics either during the washing or dry cleaning, they get damaged and stop working.
There's always the throw away hand warmers, foot warmers, body warmers, etc etc and I know those help out tremendously.
Another thing you might consider is a Blind with a heater.
I use the "Doghouse by Ameristep" you can easily fit 2 people in it very comfortably or you can get a one person blind.
This is the Doghouse model. The windows on all sides are zippered so you open them as much as you want,
you can either use the full window or port holes visible in the pic.
Good Luck and Welcome to the Club.
Democracy is two wolves and a lamb voting on what to have for lunch.
Liberty is a well-armed lamb contesting the outcome of the vote.