we have some downdraft tables at the shop
they just collect dust.......as in we dont even bother with them
but if you do build one get a roll of drawer liner for a tool box like this
http://www.lowes.com/lowes/lkn?action=prod...&lpage=none
to pad your top. your work wont move on it and the holes will let the dust thru
its what i used to use on my bench for sanding now i use wet suit material as it lasts longer
but it does not have any holes in it