Estes Constellation, Part 8, Airfoiling The Fins

I used the rounded side of my old sanding block with 400 grit to sand the recess at the rear trailing edge.
This seemed to be the best way to get a round concave shape.

You could also use some 220 grit sandpaper wrapped around a 24mm engine casing for the round contour.

With the trailing edge facing up - 

Here's how that sanded recess looked after sanding.
You can see how the trailing root edge tip remains thick and less likely to break off. I still have to sand a "knife edge" to the remaining trailing edge.

With the fin held upside down,
On the left is looking directly at the root edge. The tip was rounded off.

On the right is the tapered trailing edge viewed from the rear of the fin.