I use a similar solution to some mentioned above, except that I ended up simply using heavy cardboard attached with adhesive backed velcro. I intended to replace the cardboard with hardboard once I got my design right, but it worked so well that I still use it today many years later. I closed up only maybe 80% of the back to allow for airflow. I had the front slot also covered with a piece of cardboard when I was using a shop vac for DC. However I recently upgraded by building an overarm guard and pull the sawdust from both below and above using a Jet DCK1200. It works very well. Should have done this years ago.
Good Luck.