I agree with Gary, I think most fillets should get finished off with your finger. It not only gives a nice looking consistent fillet (and a messy finger), but pushes the Hysol into all the little cracks and crevices for a good bond. Alcohol does a great job of cleaning up unwanted areas, fingers, etc.
I personally wouldn't spend any time squaring off that triangular area on the bottom of the rudder. Its not seen, and the fabric will conform fine to the triangle.