Check it out - I finally got inspired to build a downdraft sanding table (after reading one of Hesh's posts on his recent shop visit). It has a 30" x 48" table surface (1/4" pegboard on 2x6 frame) on a cabinet that has inside dimensions of 25" x 48" (so it holds 3 of the 16"x25" air filters). Took me the entire weekend and ~$350 (half of that is for the fan), but it is really doing a great job keeping the dust down. I'm spending the afternoon today reaming out the pegboard holes slightly.
The switch turns it on/off and the outlet is handy for the sandy. 5" casters (2 locking) so I can move it anywhere. The front edge of the table extends about 5" out from the cabinet, so that I can clamp to it. I made sure to plane and thickness the 2x6s to uniform width so that the table is flat.
The table is hinged to the cabinet so you can lift it and access the filters (here I have some cheap fiberglass pre-filters on top of the better white folded filters). Foam rubber weatherstripping around the top of the cabinet where the table makes contact - no leaks!
The entire inside is caulked so that I don't lose any airflow - I want the fan to pull air only through the table. The fan is a 3000 cfm wall exhaust fan, and it is amazingly quiet.
Even when I'm not sanding, I'll turn it on just to circulate and filter the air in the shop; it will circulate all the air in the room every 2-3 minutes.
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It should do a fantastic job sucking up whatever dust you can throw at it.