under rear bumper would be pretty cool...untill you needed acess to it.

I think you'd need to cut into the bumper support, and if you get rear ended...structural integrity wouldnt be as good.

I'd personally say invest in a dry cell battery and put it in the spare tire well. Dry cell, no gasses, can be tilted with no spillage or leekage. A bit pricy, but its worth doing right if youre going to do it.