We initially considered exactly that - a styro cup mounted atop the turret - because it presented the same target profile regardless of the view angle. However, being able to knock it off meant the cup will likely fall off when the tank rolls over uneven terrain. And as for getting damaged, we could hardly get a dent on the lowest quality cup available using the best stock tank gun we have (a Marui tank gun that shoots well over 25 meters). We couldn't even shoot through a much thinner McDonald's styro box. That's why now, for assured shot penetration and possibly more dramatic effect, we're considering card thin wafer or baked crust.