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The back of the duct should have a rounded entry, rather than sharp, the dimensions of which are specified in the NACA paper. You can use balsa wood, but we had some fir lying around and justified its use by greater resistance to denting. We cut it slightly longer than the width of the duct and glued cardboard profile templates to the ends. We then shaved the wood pretty close to the required shape. It's easier to take more off in the next step than to have to fill and sand again.
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