This allows the official Samsung 3000mah battery to fit perfectly into the Slim Armor case. The case looks similar to the Slim Armor made by Spigen for various phones but this particular case is not made by Spigen.
Things you will need:
3000mah extended battery
Slim Armor case
Dremel or Stanley knife
Measure or trace position of battery on origional S3 battery door and cut it out with a rotary tool or Stanley knife.
Lay battery door inside the soft part of the case and cut out where the battery will go.
Install new battery and put on battery door and assembled Slim Armor case.
The gap between battery and case is perfect but if for some reason you have some wriggle room put a layer or 2 of soft tape inside to close the gap.
Hope this helps people like me that would like some extra battery life without turning your phone into a landmark you could spot from space.
I will try to upload some photos if I figure out how.