Got to thinking about this and although I agree you can't have a projectile flying around to injure (kill) a spectator. I think a good solution would be an impact - rollover switch which would shut things down immediately and I plan on looking into it. My Sonoma's cut off switch can be activated from outside and inside the cab but that doesn't do any good if your not conscious to activate it. Food for though.
The hood or Trunk lid will keep the battery from potentially killing anybody. My battery was 150 ft BEHIND my car, so the car itself would have killed many more potential spectators that where seated on the outside of a corner, for some reason. A battery isn't like a tire or a hood. its only going to go 20 feet or so from where it comes out, if it comes out and they don't really bounce too high.
I'm not saying don't secure the battery, I'm just saying the mounting doesn't have to withstand 5G's and definitely not 20G's.