I don't know of a trick to make an edge rounded, just some elbow grease will do!

For the skin on the fin and stab I use a metal ruler and a sanding block to make a 1/64" indent in the balsa skin.

Glue the 1/64" ply skin in place with medium CA.

When I glue the skins on I left a 3/16" overlap over the hinge area.

last picture shows the stab, notice that the hing point is set at about the end of the 1/64" plys edge. Also notice the centerline drawn on for proper hinge alignment.