Ok, so we need the following:

PIR circuit: that's just send 5v, and returns high/low to input

Light sensor: that's just send 5v, and returns high/low to input

IR illumination: we switch this by sending high/low from an output. Since it is 12v, and the Pi is 5v we use optocoupler (e.g.  Isocom 4N25 DC Input Transistor Output Optocoupler, Through Hole, 6-Pin PDIP £0.20) , and we need transistor too.

Can we put all these onto a HAT? I guess so!

We also need a stepdown circuit for the 12v battery to 5v for the Pi. Can we put this on the HAT too? Like AmpZero did, so we power the Pi via GPIO.