Utterly Mad

But hey, the cast iron trench armor is still a possibility, right? :|

Uhhhhh. Trench armor, possibly. Cast iron? Probably not, since it's a real pain in the ass to keep unrusted, and it'd be too heavy and obscenely expensive for something as simple and silly as an airsoft tank. The heaviest "armor" I would go with is probably plywood around a steel frame, since plywood is cheap, far easier to work with, and doesn't rust. I'm protecting the crew from BBs, not (real) HEAT rounds.