How to make an efficient small scale heater powered by a battery?
The specific heat of an 18650 LI Ion battery is 0.83 J/gK.  Weight is 46.5 g. To convert mAh to Joules, you need to know the voltage.  If you are working at 3.6 volts, 500 mAh = 1.8 watt-hr.  Since a joule is 1 watt-second, multiply by 3600 to get 6480 joules.  You need 0.83 * 46.5 or 38.6 J for each cell to raise the temperature 1 degree K (or C).  So 6480 J would increase a single cell's temperature by 167°C in a perfectly insulated environment.  The addition of an aluminum surface just adds specific heat, so you just need a little foil to prevent a hot spot.  Just insulate the batteries well and put in whatever type of resistive load makes your batteries happy.  Efficiency will be a function of the insulation; essentially all of the energy will go into heat.  Of course if you use a small load it will take longer and your imperfect insulator will cause a loss of some heat, so you don't want too small of a load.  Also don't let the batteries get too hot.