If you'd like your chowder even thicker, try adding a couple additional tablespoons of flour before adding the cooking liquid. Or if you have already made the recipe and feel like isn't thick enough for your tastes, you can stir equal parts cornstarch with cold water (I would start with 1-2 tablespoons of each) and mix well. Stir the slurry into your chowder at a simmer and it will thicken up a bit more.