The old fashioned way...
Just wrap the stuff up in tinfoil and set it on the Jeep's engine block for a while. Of course, if your units has no Jeeps, tanks, or other mobile units, you're pretty much screwed. What's wrong with solar heaters? A properly-designed parabolic one would not cast tell-tale reflections in all directions on the ground. Would make a pretty bright spot from the air, though...