You can buy little hand warming packs that use oxidizing iron as a heating agent, no flame, no odor, no mess. They last several hours and I use them in my gloves when I fly in Winter. However I have also tucked one away in the fuselage and it indeed kept the insides warm. I have reservations about recommending but I don't believe it will harm anything as long as it is secured properly and kept away from direct contact with equipment.