BLW sent me a quick email with a link to this thread (thanks!). In a nutshell, saramos got it right, I didn't use a published airfoil. The guys at Sig used to call this an "FCR" airfoil - french curve and ruler. The main priorities were the flat section on the aft bottom portion (for ease of building) and efficient use of balsa. Gray Beard gave good advice, but I think it's a little thicker, like 14-15%. Maybe a NACA 2415 would be a slightly closer approximation.