If you know from the start that you want an extra thick clam chowder boost, the butter and flour in the white sauce to 1/2 cup each. If you want to thicken the chowder after you've placed the white sauce in the Crockpot or slow cooker, blend 2 tablespoons of cornstarch with 2 tablespoons of water in a small bowl, stir until combined, then stir the cornstarch mixture into the chowder.