What I do is use those recharagable heaters used for out doors men. You boil it in hot water til the crystals melt, then when you want heat flick the corner of the disk and it will go from a liquid to a solid and make a hell of a lot of heat.
Of course I got the Enginer kit to supplement my heat in the winter. I just route the "exhaust" from it and the IMA system to blow inside the cabin vs outside the rear bumper.
