Hand warmers I've used them before. You can buy disposable ones from a pharmacy but they also do reusable gel ones too. www.amazon.co.uk/d/Hiking-Hand-Foot-Warmers/Magic-Gel-HW-001-Reusable-Heat-Pack/B000MU4IJ2 It's not an advert it's the first one I seen But they will work. Just use a bit of cotton or other fabric to place the heat pad into before application to phone