anywhere, just put anything that takes up more cubic feet, it should bring your tuning up a bit. I'd say add big 45 degree corners like biiig and chop off 5 inches of port if you can do so and add even more bracing. to calculate the displacement of the 45s, you find the volume of a triangular prism and to find the volume of the 2x4s you find a volume of a rectangular prism if you wanna be precise.
Formula for 45s 0.5 width x height(include wood thickness) x length = answer divided by 1728 to get cubic feet. For a rectangular prism, width x height x length = answer divided by 1728
You'll want to take up at least .4 cubic feet of displacement because when you make the port shorter you gain cubic feet back also. We are shooting for 35hz and up tuning with a net volume at 2.5 cubic feet