Lots of really good setups from everyone. I'll go ahead and add my somewhat unique approach using matched twin high-capacity industrial fan units, described in this post from 3/31/2010.
It's worth noting that I live in the Puget Sound (Seattle, Washington) area where the summers and winters are relatively mild. This means I almost always have a pleasantly cool pool of air available right outside for my direct ventilation needs, even in January. Don't know how well it would work if the temps were like Phoenix, Arizona.
Like you, I also boarded up an existing window, then mounted my fans inside a completely enclosed external box outside the house. This keeps down the noise considerably. For those occasional very cold winter days (it was -2F/-19C one night a few years back) I also installed a light bulb socket inside the exterior fan enclosure, with a wall switch inside the darkroom next to the speed control. I can use this to keep the fans warm and running smoothly when it's so cold outside that I only need to run them at very low RPMs.