I use Apogee's Epoxy Clay and like it a lot. Roll a thin little sausage of the clay, place it at the junction of the fin and body tube, then drag a popsicle stick along the length of the clay and fin, and perfect fillet! The only hard part about using it is that it's very sticky so you have to keep wetting your finger.
Tim is right in his ad, though, I always use less than what I thought I would use. Really good stuff.