Sighard Hoerner's 'AERODYNAMIC DRAG,' also has material dedicated to various 'wing tips' and their respective drag coefficients that might be insightful.
Hoerner worked at Messerschmitt.
He personally aero-modded a 1935 DKW 'Reichs / Meisterklasse' two-door sedan, dropping the drag, from Cd 0.64, to Cd 0.25, using the Jaray 2D shapes you're working with.
