Just a tip, on my Pearl using the standard OEM battery I would store three MicroSD chips inside a thin plastic envelope and lay that flat on top of the battery.
I would then remove the little square foam that sits in the middle of the battery cover.
Was a great way to carry three additional chips on the phone. You can store less than three, obviously.
Also, the chips acted as a good way to add pressure against the battery door, thereby making it a better fit [that is, you can do this in lieu of a folded business card or whatever].
On my Seidio extended battery [which I use exclusively now] there is no room between the battery and the cover within which to place the chips. But I discovered that there is a little space that allows me to store up to two chips, coincidentally right above the Pearl's MicroSD slot. See pic.
Just some thoughts.