Your SIC board comes equipped with a standard 10’ US Fin box. The fin box is designed to accept the stock SIC fin as well as most aftermarket fins. Your fin should fit tightly into the fin box.
To install the fin, lay your board down with the EVA facing down. Remove the aluminum plate and screw from the fin and slide the aluminum plate down into the fin box and into the recessed track. Position the aluminum plate as far forward as possible (for access later). Set the fin into the fin box by sliding the brass pin into the opening until it bottoms out, then slide the fin as far backwards as you would like. Next, push the front of the fin into the box and attach the fin to the aluminum plate with the provided screw.