Hello, I recently purchased a Harman P38+ that will be installed below grade. I am going to install using a clean out tee, a 6' rise and then 2' through the wall with the wall thimble. My question is how do I need to install the wall thimble. I purchased ICC EXCELPellet vent piping. The installation instructions allude to a rough frame to mount the thimble. I'm okay with doing that but then what does the outside flange get screwed too? I have vinyl siding. Does it go over or under the siding??Any help will be greatly appreciated as I am very new to the pellet game. Thanks.