I don't have a better shot of just the air box. Actually there isn't a box. I just cut a piece of aluminum that fits in the area where the battery used to be. It is straight along the top front and bottom, and curved along the back edge to match the fender well. The top bolts behind the fender lip, the front to the radiator support and the rear to the wheel well. This basically seals the area behind it. There is a 4" hole cut in it and a 4" dryer vent flange on each side.
The filter is on the sealed side, and the air tube on the other side leading to the engine. The second photo on the webpage gives a better view of the shape ot the block off plate. It angles into the engine compartment slightly from top to bottom to allow more room for the filter behind it. The air flows to this area from behind the headlight.
You could use 3" pipe and do the same thing going to both sides, one from each butterfly.
One hint, glue the pieces together on the inside in several spots to hold it together, then use the PVC weld to seal it all the way around the joint. Then sand to a smooth finish.