I have the anker 1900mah. It is a tight fit. I didnt have any problems with it but case/cover extended battery are a more practical way to go.
Just buy a case/cover battery from ebay ... it will add another 2200mAh to your existing battery.. .while it is bulkier, it is not that unuseable. It will keep your phone powered for at least 8 hours under intense usage.... (a lot more hours if you do not much)... When the case battery is drained down, you can easily remove it and have your old phone back.
But to extend the life of your battery on your phone, you are better off running it down to 40% before attaching the extended case battery. The case battery will automatically charge your phone battery back to 100%, while it gets drained down. And itself will run out first. This way you minimize the time spent with the bulk.