If you look around, there is plenty of free coroplast these days, now that the election is over! I'm not sure how useful the 4mm stuff is, but I plan to give it a try. I wanted to build a mugi, but 4mm might be too thick...? In any case, I'm sure I'll make something really fun out of the stuff.
This might be fun...

http://youtu.be/zTrwJKlgwSU
Cheers,
Andrew c