Back in May, I made my first post about a set of window fans I’d built to deal with the Seattle summer weather in a rental with no whole house fan or whole house AC - these gizmos.
(Comments from Blogger)
2015-12-26 by Mike Halcrow
+1 for using duct tape to make a duct.
2015-12-26 by Robert Moser
Glad to see it worked out well. And that duct tape lived up to its universal fixer reputation. 
Not that you’re looking to replace the fans, but a friend of mine recently told me about these:
http://www.amazon.com/Noctua-NF-P12-Nine-Blade-Bearing/dp/B002CQU12C
I’ve used a few in some PC builds, and those notches make a significant difference in their acoustic signature. You can still hear them, but the quality of the sound is much less ear-filling than the white-noise of standard fans.
2015-12-28 by Amy K
For the fan, why not use something like this:
http://www.amazon.com/Holmes-Blade-Window-Touch-Thermostat/dp/B000065DK8/
We have two and really like them. I like the thermostat control because it can automatically turn on when it’s too hot, off when it gets cool overnight. The rigid plastic extenders should work fine in the sliding window - some of this style of fan only have accordion extenders which won’t support any weight.
2015-12-28 by Russell Graves
I looked into those and couldn’t find any sort of flow rating or static head rating on them (how much pressure they can build up). Given that I’d tried box fans and similar previous years, and they couldn’t fight against the prevailing winds, I knew I needed something a lot more aggressive. So I built it.
For most homes, something like you liked will probably work just fine.
Also, I had a need to over-engineer the hell out of a solution, and this one got picked. 
2015-12-28 by dewey
Isn’t the window A/C basically an open unit inside meaning that any negative pressure from inside will pull air through A/C rather than your filter unit. I am guessing that is why your filter unit was so clean after a season of use.