I'd love to do the same thing. Looks like plenty of space in that plastic compartment under the hood (drivers side). Actually...looks like it was intended for a second battery. Anyway, its there on my '05. Never was sure how I'd hook things up so it would stay charged, but could disconnect it easily so when the car was off...one battery would stay fresh while I drained the other on the stereo. Or something like that.
Oh well, as it is, I just don't get to listen to my car stereo on the beach like I used to.