I use my index or pinky finger for fillets for I have much better control than a popsicle stick or similar. Also by the angle you use you can cover more or less of the joint. I will do fillets where the rib meets the shear web on one side of web, ot both or where formers meet fuse but thats about it. I don't recalll if the 4star 40 vertical tail has tri-stock at the base or not but for planes that don't have any support, I will usually add some small tri-stick in place of fillets at that location.
Mike