The drag coefficient of a long narrow rectangle is about 1 giving about 2.5 lb of drag at 40 mph apparent. Simply rounding the leading edge will reduce the drag coefficient to about 0.5 and making an elliptical plank section will get you down to about 0.1 or 0.2. I strongly recommend rounding the leading edge.
Elliptical sections are pretty easy to do.
Full airfoil sections are not worth the bother. Overall the plank is one of the bigger aerodynamic factors that you can do something about. Bob Cummins suggestions about clothing are right on the money as well. They work very well on biplanes as between the wing tension members. They do not work well on boats for several reasons.