I have been using this method for many months and it seems to work well so, I thought I would share. When using the overarm guard, the saw looks like so:
Although Saw Stop has made swapping the guard for the riving knife and act of simplicity, where do you put the arm when it is not in use? I stick mine on this cleat fixture which I can reach from the saw's operator position:
But, that leaves the overarm tube open. I used to use a little mini Nerf football to stuff into this hole but, had one of those ah-ha moments. I took a rattle-can top that happened to be sized so that the inner cylinder fit inside the SS overarm tube. I slipped an O-ring over this inner portion and stuck a magnet to the inside of the outer wall.
Now when the overarm is stored, the cap fits on the tube making a reasonably airtight seal. When not in use the magnet in the cap keeps it stuck to the frame or tube nearby and below the table surface height.
Stupid-simple but, it has yet to give me any grief when swapping back and forth from over arm to cross cutting or sled use.