I saw lots of workbenches with built-in downdraft sanding areas but for the small shop people we needed a simple box that can be put away when not in use to recover bench or shop space for other tasks. This seemed like a much easier design project than it turned out to be. Getting the inner braces shaped right to maintain good dust flow across the whole box and thereby good suction all of the board took a bunch of trial and educated error.
To make the Downdraft Sanding Box more usable when sanding on vertical surfaces we added easy to remove legs that let you stand it up next to where you are sanding. It certainly is not 100% effective but it is a bunch better than a fan or hoping for a cross wind!