I have always used layers of window screening.
I start off with 3 layers (( supported by egg create )) and remove a layer every 5-7 days. Never had any ill effects with existing corals/anemones in the tanks while doing this.
This is what I am talking about (( ignore the oversized light, hadn't gotten my new light when this picture was taken --- same wattage, just a smaller fixture ))
While I still want to take pictures of this over the tank, the following pictures should give an idea of how much it will block.
3 layers
2
1
And this is a picture of just the egg create --- this was for my 33 cube, the cut outs in back were for the overflow and return pipes.