Hi Brian. Your plan to invert your sub will make it necessary
to use AGM batteries or maybe the Optima sealed ones so they don't
spill. There's no way to add jello to a wet battery. Tesla motors has set
up a battery bank made of a few thousand lap top batteries, but that's very
complicated and expensive.
The AGM batteries can be turned on their sides for short periods, but not
upside down of course.
I had a dirt bike years ago with a gel-cell battery, but it was designed
to resist vibration and rough riding. It could be turned side ways and even
upside down for SHORT periods of time. Frank D.