Circuit Board Design Help (Novice)
I'd use a PIR (passive infrared) sensor.  These are amazingly sensitive to IR emittance changes in the environment.  They don't take much power, so can be battery operated.  The electronics should be able to fit on your board easily.  The real problem space-wise is the power source and whatever optics you put in front of the PIR sensor, even if it's just a slit baffle.