Or use a sealed box as legally required if you have it in the boot.
as for the drop tank, to remove the wheel well and reinforce the area, then have the drop tank installed and engineered . it will cost you a fair bit.
have thought about this before though .. you also shift the balance of the car since the std tank is over the rear sub frame, this would move the weight back and potentially cause some over steer characteristics when full tank and cornering