You can buy 8"x8" Doran darkroom louvres online (got mine from bhphotovideo.com I think), and that's exactly how they work. One thing to keep in mind is that because of the design they do restrict airflow to about half what you'd expect from that area. In other words, one 8"x8" Doran louvre is about the same airflow as an unobstructed 4"x8" opening, or 32 square inches. Since you want your input area to be 1.5-2.0 times the size of your exhaust hole, you may need more than one darkroom filter to provide adequate airflow. If you use filters over them (which I recommend, to keep dust and crap from entering your growing area), this restricts airflow yet more. I don't use these darkroom filters on my exhaust (since it's lightproof anyway, with ducting going to it), but I found I needed two of the 8" x 8" darkroom louvres to provide enough inbound air (using a 6" centrifugal fan and thus a 6" exhaust). Your mileage may vary, of course. I'd recommend just getting Doran's, since they have enough bends to insure lightproofness, but no more. In other words, they've been doing this long enough that they have maximized airflow while maintaining lightproofness, whereas if you build one from scratch you will be guessing and probably won't achieve that efficiency.

Disclaimer: I do not work for Doran.