I decided to lay it on the battery. Right in the middle. I am able to put the back on and it does not bulge.
That's not exactly a brilliant idea. The heat from the battery could very well melt the very thin plastic on a MicroSD card and/or damage the flash cells inside.
You'd probably just be better off getting a higher capacity card than trying to carry a spare somewhere in the battery compartment. The Storm should be able to handle up to at least 16GB cards, but I wouldn't be surprised if it could handle more.