I have a small workshop and it just so happens that the furnace is in that room as well. You can well imagine the dust that gets into the furnace and goes through the house, especially when I'm using a power sander.
Step 1: I Used
Keep in mind that I may have gone a little overboard when I built this, but I wanted it to be durable with a work top that would put up with years of use. With that in mind, I used 3 sheets 1/2" plywood, 1 sheet form ply (pic 1, 3/4" has a coating on one side) 3- 2"x2"x12", 2- 1"x2"x12" strapping, furnace blower and motor, 14' 1/8"x 3/4" angle iron, 8' 1/8"x 1" angle iron, 6' 1/4"x 1" flat bar, 9 furnace filters (3 each of 800 MPR, 1000 MPR, 1900 MPR), 4 T hinges, and lots of screws. This build wasn't cheap, it cost about $450 with the filters being about 1/3 of that but it should get 90% or more of the dust before it has a chance to get into the air.
Step 2: Cut Your Material
I wanted a box that was 3'x4'x3' high so I cut the floor 48"x35" to allow for the plywood sides, 2- 48"x 35 1/2" and 2- 35"x 35 1/2" for the walls. For the sharp-eyed builders, the 48" length for the floor gives me a finished outside length of 49" but I wasn't about to cut 1" off the stock-cut plywood. The odd height (35 1/2") is because I wanted casters under my box and I only wanted the sides 1/2" off the ground. The slats for the top are 1 1/4"x 35 5/8" - wait until the steel frame is made so these can be cut to proper length allowing for angle. Pic 1 shows the sides with the 2x2 ledge that the floor will sit on, pics 2 and 3 show where the casters fit. Once the main box parts were cut, I put the 3 sides on to make the end where the blower would sit so I could build the enclosure for the blower and frames for the filters in place. I used as few screws as possible to keep things together as I built it because I had borrowed a shop so it would have to be dismantled and moved.
Step 3: Re-assembling the Down-draft Table
I recommend using a furnace blower if you can get one, I bought mine used from the local heating/cooling guy in town. Pics 1 and 2 shows how the blower fits in the enclosure and the frames were placed for the filters The black in pic 1 is 1/2"x 1" weather stripping. I also put 2 strips on the bottom of the blower all in an effort to minimize vibration - it worked. The filter frames are 3 1/4"x 32", 25" and 16" with 1"x 1" screwed to the frames and the enclosure because the filters I used are 16"x 25" and 3 filters thick. Be sure the openings in the enclosure for the filters are at least 1" smaller than the frame to hold the filters. Pic 5 shows the steel frame for the top. 3/4" angle for the outside, 1" angle for the 2 cross members, and 1" flat bar for the span across the center. I pre-drilled the holes in the angle before they were welded together then the wooden slats were all screwed to the 4 angle irons to make it all solid. The slats are cut 1 1/4" wide and there is a 1" space between each for good air flow. The top was mounted to the box with hinges to make it easy to get at the filters when the time comes. I may add cotton towels or a sheet over the filters to catch some of the dust so the filters last longer. Don't laugh at my bungee cord filter holders, again for ease of access. A bit of math for you: This blower came from a furnace that used a 16x25 filter = 400 sq in. I have 3 filter sets = 1200 sq in. The top has 1" spaces x 21 = 21 x 36" = 756 sq in of inlet. With the extra filters, they will still have excellent flow even when half plugged. With this build, I can feel the air being drawn into the box so I think it works well. If you make the spaces too small, you restrict the in-flow of air and it won't efficiently draw the dust in.