My radio aerial is at the back, in the roof, so I put the flettner just to the front of that. Before I started, I made up a marine ply wooden spacer to take up the gap between roof, and the liner, and painted it, making sure to do the edges really well.
I Then used a circular hole saw to cut the big hole in the roof (worst moment ever starting to drill a hole that size in the roof), then once the hole was cut, I painted the edges, then used silicon sealant to fix and seal the wooden spacer to the roof. The 3 screws that hold the vent on, go right through from the top, to the bottom, so once fitted the roof, spacer, and roof lining are all sandwiched together, between the inside vent and outside rotatory fan. The inside vent has a slide switch to open and close it, but so far I have never needed to shut it.