On Camaros, at least, there is a body panel just south of the front wheels on both sides of the car that bolts into place where the jacking mount is under the car. The body panel is on the side of the car though and ahead of the front door edge and under the wheel fender.

Does anyone know what I'm talking about?

I saw in a Car Audio magazine, about 8 yrs ago, where someone had mounted an extra battery behind this panel.

Has anyone tried this? Is it hard to do?