Cars that have them in the boot dont normally have room under the bonnet.
Unless its for a clean looking engine bay its pointless.
Far from pointless it actually changes the weight distribution quite drastically and moves a load of weight from the front of the car to the rear which in theory should help with the overall balance.
It needs to be mounted either in front of or above the rear axle - putting it behind would increase the pendulum effect at the rear. You mention GTVs, they have it tucked up under the rear parcel shelf in just about the most awkward place
I guess that ideally you'd want it as low as possible and as near to the centre of the car as possible - behind the passenger seat maybe? Make sure that if you locate the battery inside the car it is properly secured and vented.
Make sure that whatever cable you use is properly shielded and protected from rubbing against the bodywork or things could get messy very quickly.
