for easy external battery, look at the anker astro 5600 on amazon $41 3.5"x1.7"x.86" this comes w/ short charging cables and you can buy a small sub-min camera case for under $10 on amazon.
the 5600 mamp capacity puts out 1amp at 5 volts just like the oem samsung charger that comes with the phone.
there is one oddity. the nexus recognizes the usb cable as if you are charging from a computer usb port and and will only let the nexus accept 400 mamps rather than the full 1 amp that the battery puts out. this problem is also solvable:
monoprice3--sells a retractable micro usb charging cable the data pin are shorted-out so the the nexus will accept the full 1 amp charge and charge from the anker astro 5600 as fast as a wall plug oem sammy charger.
For only $1.25 each when QTY 50+ purchased - Retractable Micro USB Charging Cable | USB 2.0 Cables - Micro-B Type
with this set up, i can fly all day long and half the night and have juice left when i arrive--watching video, listening to music and burning up power on angry birds