First, I don't like to leave my fin edges square. Even if I don't sand in some fancy airfoil shape, at least I round the leading and trailing edges.
Which may or may not make any difference in the following.
If I want to make a fancy fillet, I don't use glue or epoxy. Glue is too erratic in its behavior when drying, as in, it usually dries/shrinks into some goofy shape that I didn't want. And it is too tough to sand easily without damaging the surrounding cardboard or balsa. Epoxy is too heavy, too brittle, and too incompatible with too many primers and paints.
If I want a fancy, nice-looking fin root fillet, I break out the light-weight wall spackle, the kind that has the glass micro-balloons mixed in, where the full tub on the store shelf feels empty when you pick it up. This stuff mixes and thins with water, so you can flick a few drops onto the top of the tub and stir to re-activate a dried-out tub, or wipe with a wet finger to reshape a fillet. It sands very easily, and when you get the shape you are happy with, it absorbs CA quite well, and primer sticks on top very well also.
I just shape it around the front of the fin root....but mine are usually more round.