..The D-tube leading edge is commonly used to provide a stiff torsion resistant structure for aircraft that will be experiencing a lot of force when flying. Here's two methods of construction..
The first, with the full-depth spar and ribs eggcrated together is the stiffest possible, with a minimum of material.
The second is the usual spar and vertical web found in many kits. Easier to construct than the first example.

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