Got tired of trying to find exhaust vents for a 4 in duct system that didn't contstrict airflow for my bit minders. Same situation for the intake. I used PLA for all of these and piece them together with window screen replacement from Home Depot it the middle. I used #8 -32x3/8 in screws and nuts to piece together. I am working on building more to put in the window for intake and exhaust. So far they are working great for air flow. I have also included the duct adapter to mount on your fans as well.
If you are using these where sun is going to hit them, make sure to use white material.