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Mine'll be hard to miss:

I cosplayed Kaku from One Piece.

...or if you saw a wayward giraffe in a black track suit wandering the AA on Saturday.
I got photos from the two fancy-pants photographers, but would love to see photos from fellow con-goers.  ;D

Oh man, I have so much experience with building sculptures and the like out of cardboard. It's kinda hard to explain some things, but I agree with many points Xgouki has brought up.
One question: How large are you planning on building it? Like the actual size from the anime kinda...size?

Considering you have something to reference from you don't really have to wing it with designs. But just because you have a reference, doesn't mean you won't need to wing some parts.
Since you're going to be painting it, you don't have to worry about where the cardboard comes from or what is on it. I used cardboard from various sized boxes around for the stuff I made. But considering my question of how large it'll be, you might need to scout out grocery stores, or even Costco if you have one around, or the like for large, flat, unbent cardboard for large areas that should be one piece (like take for example, refrigerator boxes).

If you need some areas to be strengthened, hot glue some cardboard to the back of the panel, and have it overlap that part. The more surface area/edges that have contact with other parts = strength, like in all crafts, like welding and wood working.

If you still aren't sure what to do, you should probably contact the people who did the other Lagann and ask for tips from them.