I didn't use the GTM rubber D section they supplied, I didn't think is was big enough to create a decent seal especially at motorway speeds. I used 8mm thick adhesive backed closed cell foam of a good quality, and stuck it to the underside of th e roof panel, you can mark from below with the panel fitted to install the foam perfectly.
I can't feel any air entering the cabin at speed, mind I don't drive the GTM when it's raining especially now when there is salt on the road, you guys are cruel
I Don't know if you have the standard roof fixing balls, but I bought larger diameter ones you can get a better grip on them for tightening them and undoing them they never come loose.
Jamie