2x4's obtained from the local lumber yard. It is one of the lightest types of wood available with high strength.
Mahogany is heavier, but also a good material.
Plywood is bad because some of the laminates in the plywood are oriented the wrong direction for the loads on a centerboard or rudder. This step took a fair amount of time, but it enabled the final shape to be easily obtained with a plane or sureform in very little time. I could make a centerboard in a week or so of working evenings. Let's hear more about foam sandwich construction.