Tom here is another way to make that "flashing" you are refering to. I have used this method before and attached are 2 pictures from my Pilatus PC-7 flap. Also a drawing with a starting image and a finished product. This way looks a bit better than Larry's but may take a bit more time.

Attach a piece of triangle stock to the top of your trailing edge. Then take a nice big sanding bar and sand down the top to match your airfoil. Then take a dowel the size you want your radius to be. I think the PC7 was 1/4" dowel. Glue a piece of sandpaper to it and use that to create a grove in the bottom. This will make a very nice flashing for you. You want the dowel to be about as big as the flap area so it keep it all nice and even across the span for the flashing. Don't use real soft balsa so it holds up a bit better where the wood gets thin use some thin CA to harden the edge. Once the flap is installed it will be quite hard to dent since the flap protects that edge.