You didn't mention the sub box. That sure changes things. I know of no other place in the car you can stash stuff.
Maybe you can leave the flask at home? I sure don't want to push morality here but if there isn't a safe place to stash it I'd be inclined to leave it at home. At least if it was found under the floor cover in the hatchback they can't say you had an open container "in" the car. It was put away for transport.