Pi Zero battery backpack case using a Samsung S5 Li-ion.

thingiverse

I believe this presents one of the highest compactness to battery capacity ratios for a pi Zero, which is extremely ideal for numerous projects. You can acquire HATS for AAs or other LiPos, but nothing surpasses a good old widely available smartphone battery. Especially since it occupies little space around the Pi. A 100mAH battery lasts approximately 20 minutes running the pi Zero, so 2500mAH should last about 7.5 hours of constant draw. Samsung S5 Batteries are very reliable and pack 2500-3000mAH of charge. You can find many aftermarket ones; the OEM is $11 CDN on Amazon. Because it's 3.8V you're going to need some sort of 5V voltage regulator. I would suggest an Adafruit 1000C. They're pretty small, so it wouldn't be any more significant space. Mounting one to one of the Pi's corners and directly to the GPIOs would be the best bet. The print in the pictures was my first version without the ledge, so I melted a ledge before updating to the current CAD model. The heat gun melted a little too much on it, but oh well; the new one works perfectly, no melting required.