The shiny rubber seal right up against the radiator fins is black silicone rubber tubing. I purchased 10' of it from a seller on Amazon. It's very soft and pliable and stands up to high temperatures. Using a box cutter, I split the tubing from one side to the middle. This allowed be to place the cut tubing over the fan shroud plastic edge creating a nice seal. It was only 10 bucks for 10' of tubing.