From what I have read one layer of regular cheap fiberglass screen from Home Depot or Lowes would reduce light about 40% if I remember correctly. I think Hoppy had a thread where he actually measured this with a par meter, you could search for that.
If it were me I would take the 2 screws out that hold the endcap of the light on, the polycarb lens/shield on the light just slides out of the end. Cut the screen a bit smaller than the shield, slide the shield & screen back into the fixture. Your top can still be open or glass without seeing the screen, but it is in the light to cut down PAR.