The product you are looking for is "Raingo" down spout made by Genova Products. Call them at (810) 744-4500 and ask where in your area you can buy the Raingo 2 1/2" square down spout. It's sold in local hardware stores in 10 foot lengths but relatively inexpensive. You won't find it at the "Big Box" stores.

The flutes in the coroplast material should be oriented in the chord wise direction and the wing should have a spar in it for strength. I have used yard sticks and 1" by 1/4" wood strips that I find at Home Depot. Look at the "Spad to the Bone" web site for more details about building planes from coroplast.
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