I bit the bullet today and went out and bought a 75AH Deep Cycle battery. I already have a DC-DC Power Adapter. I've decided to go a bit further afield tonight to a lookout a few km away. I should be able to see the moon coming up over the ocean that way. I also figured it would come in handy if I am not able to get power out at Barambah Dark Sky Camp. So power is taken care of.
Fortunately it will only be operating my laptop and maybe one dew wrap if needed (T Oaks controlled)