Those Prinsu instructions are great. If all you need to do is tighten it, you can just pull the top trim and an upper side panel. I had to take two screws out of the lower side panel to loosen it enough to pull the upper one out. One screw is above the rear cup holders and the other is where the cargo cover hooks in. Then just give the headliner a little tug to pull out the clips. 22mm nut.
Unfortunately, tightening it didn’t get rid of the gap for me. The nut was slipping with maybe 10 ft-lbs of torque. The driver backed into a garage door so maybe the bolt is bent?
After taking the fin off, I noticed the roof was actually creased around the hole and I suspect that’s what’s causing the gap.
After bending in the creases as best I could with the plastic butt of a screwdriver, the fin sat better, but not perfect. I used a liberal application of RTV around the rim of the fin to fill in the cracks.