Dunno whether you can still buy such things, but I used to use rechargeable handwarmers that had a little metal disc inside a gel pouch. To trigger the heat you just flexed the little disc, and the gel started to solidify and give out heat. They'd keep going for maybe an hour and a bit at least. They came in their own fluffy little sleeve and you bought them in pairs. When you got home you recharged them by boiling them briefly in a pan of water until the gel liquefied again. You could do it dozens and dozens of times. They were three or four quid a pair, so two pairs used in succession would keep you going for a few hours. I used mine to put in a bag with my chromatic harmonicas, which wouldn't play well unless they were warm. Before that I'd just put the harmonicas down the front of my trousers. Funny thing is that once I'd bought the gel hand warmers instead the girls at the session started to lose interest...