Piros - that DC board looks great. So simple and shape makes sense. How is the bottom contour up front? And rails?
cant remember if I showed the forum this series of boards I made:

Bottom shape and box construction works in the same way as corrugated cardboard so it is super stiff length wise. Boxes are glued in after lamination. Adds strength and makes it watertight no matter what you throw at it. And boxes are pretty long.
Shape outline is widepoint forward. Same theory as the DC board. Early paddle-in
Downwind-model: