Neil,
Looks to me that the NACA 2315 is not a symmetrical airfoil, I would, as I always do, suggest one of the lower NACA 6600 series shapes. They are symmetrical and very low drag. Also, as Neal suggest, shapes from the sail plane people are always very low drag. If you are thinking about something like the piece of 1/2 inch aluminum plate that one of the lakesters ran this year I would not recommend this approach at all, easy to build not very aero. You might look at Bruce Carmichael's book "Personal Aircraft Drag Reduction" as there are several good sections on tail fin design. Just a note regarding some of the info in Carmicheal's book, he writes about a small personel airplane that goes over 300 mph on less than 100 hp, really makes you think about what we do.
Rex