I've built a downdraft dust collector that I use when sanding/cutting parts. It works great, although I would like to see a stronger fan. Here's the setup:
4 pieces of 1/8"x12"x20" MDF
1 piece of 3/8"x20x20 C grade Ply
4 pieces of 2"x2"x10" Pine (for the corners)
4 pieces of 1/2" x 2" x 16" Foam (to support the filter)
20"x20" Furnace filter (I use the allergy filters to catch the fine balsa dust).
1 Exhaust Fan. (I found this on ebay)
1 110v Light Switch
1 Junction Box.
4 casters