The "welting" material that a lot of people have used for the bumpers is the rubber strip that fits between the base/bottom of the whale tail or turbo tea tray spoiler and the surface of the engine lid. It's much like the seal between the fender and cowl that Wavey mentioned except the spoiler seal comes in a longer continuous roll. I suspect the 930 spoiler seal is the best one to go with since it has the biggest base. That seal is part number 930.512.215.00 and the parts diagram calls it "piping"
This strip seal works good on the front bumper where you want it to be thin and hardly noticeable. But at the rear, that gap between the top of the bumper and the bottom of the light housing is pretty large. I don't think the spoiler seal is fat enough here? I recall some folks have bought simple weatherstripping material that is a "D" shape and it fits the gap well. Check out McMaster to see the seemingly endless options and then search the internet for your favorite source?
https://www.mcmaster.com/#weatherstripping-gaskets/=1bddlpe (click on the "Edge Seals" category)