PSSSSST.. Have you thought about resonance? If you make your airbox out of thin sheet ally then it'll probably resonate louder than your throttle bodies were! You'd have to either make it out of thicker material or have some sort of strengthening ribs inside.

That's something I've been thinking about for mine, and I reckon it's probably going to be best to make it out of GRP. That way it's easy to shape, and you can use a core material to add strength and thickness without much extra weight