Heat oven to 350°F. Spray bottom of 13×9-inch pan with cooking spray. Prepare cake as directed on box. Cool on cooling rack 20 minutes. In a medium bowl, pour boiling water on gelatin and stir until dissolved.
Add large holes to cake with the end of a wooden spoon. Carefully pour strawberry gelatin evenly over top of cake. Once cake has cooled, mix 2 Tbsp cool whip and diced strawberries in a small bowl. Spoon the strawberry mixture into the large holes and then spread the remaining cool whip over the entire top of the cake.
Add strawberries to the top if desired before serving.