Greetings. Today I installed a Broan bath vent on the outside wall of my bathroom. Because my siding is not flat, I had to cut out the perimeter of the vent as the instructions say. I now have to trim out the opening. What is the best way? In the second pic you see a window that has a J- channel or whatever its called at the edges of the siding. I thought thats what I could use. Any inputs appreciated.