Step 1: Get a Box
Step 2: Put Your Junk in the Box
If you'll notice, one "frame" is 1", the other is 1.5". That happens to be the scrap wood I had. Your mileage may vary.
The distance from the edge is 1" (the depth of the filter) and 1/4" (the thickness of the peg board), Again, your mileage may vary. You must use cool bolts though. The cooler the better. Mine are bordering on awesome (this improves air flow, I'm sure).
Step 3: Mounting the Fans
Notice the board is cut between the holes on the outside making the fans easy to center by counting holes and cutting them out with your favorite rotary tool. This also leaves holes close enough to the edge to screw the board into the inside frame.
I used three speed fans so I could adjust for sound and suckage. ...and they were on sale for $3. I'm sure you could get fancy and use different fans and a computer fan controller with front mounted knobs to adjust sound and suckage.
Step 4: The Guts
Step 5: Screw It Together
Here's one pic with flash and one without. Yes, it's bright.
Step 6: But Does It Work?
Actually, this is a filter I installed when I had one 80mm fan in the center. Now I have four...
The filter I chose is a 1085 Filtrete Extra Allergen. It filters Debris, Lint, Dust, Pollen, Dust Mite Debris, Mold Spores, Pet Dander, Smoke, and Smog ($10 at Lowes). You might want more or less out of your filter.
I used pegboard as kind of a "release valve" so the suction (AKA suckage) wouldn't collapse the filter or do bad things to the fans when the filter starts filling up.
I built it so the fans would suck air not blow air so only filtered air is hitting the fans (the fans won't get nasty).
I have the filter facing the room, not the fans. It just made more sense to me.
Maybe a fan guard or four would be in order.
Mounting the fans inside the box and adding custom fan guards might look better. But then again, my fans are in the back.
The filter just kind of sucks up against it I didn't use any mounting hardware.
Yes, my pics kind of suck.