How about the space on the right side toward the windshield, under the hood? Some cars have that space available.
Unless you plan it very carefully, I wouldn't recommend a second battery unless you are sure your alternator (whether stock or aftermarket) has enough power to drive whatever ICE you have and charge both batteries.
I've got two batt's with the stock alternator. For convenience's sake, I put the second battery actually inside of the spare tire. Our alternators are beasts. Anyway, I use the 2nd bat primarily for prolonged car-off stereo play. I'm using a small SVR battery, which is quite nice.
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