My particular case has hole all over the front, bottom and back, so your milage may vary. I used those as exit holes, and the 250mm was input. that way I could control a dust filter in 1 place.
This instructable is meant to be more of an overview. If enough people are interested, I'll get into how to make it a little deeper. I recognize this looks like ass, and given more time, I would have made a much nicer one. But this is the best I could do in my allotted time.
Another reason i did this was because I was fed up trying to use different, ineffective filter media. Pantyhose didn't work, nor did speaker grille material, or household pleated paper filters. All either restricted airflow to where no air was getting through, or just didn't work. Industrial fiberglass filters finally worked well, and didn't restrict airflow too much.
Step 1: Parts List
1. The fan came from here: Xoxide.com dd
2. Angle aluminum to make the frame.
3. Some flat stock right angle corners.
4. Screws & nuts
5. Piano hinge
6. A nice drawer pull
7. Some Velcro I had lying around
8. Wire or other thin media to make the grid to hold the filter media in the frame.
All this came from Lowe's.
Step 2: Frame Construction
I drilled holes along the edge and strung the and just wove some thin wire in & out. This basically just keeps the filter material from falling out.