ORIGINAL: khodges
I saw plans for one where you make the surface the size you want, using pegboard, braced from underneath to make it firm. Build a box to mount the top on, with a hole cut in the side that you can stick a shop vac hose in. Fire up the vacuum, it will pull air down through the holes in the pegboard, taking the sanding dust with it. You can make one 2'x2' and a foot deep for about $20 or so.
I made my own using pegboard, it's 2' x 3' and I use my delta collector with it (lower noise than a shop vac) one tip if you're going to do it yourself, bevel the bottom so it gets shallower on the end away from the nozzle, this helps keep airflow more constant across the face of the pegboard.