Go to any hardware store and ask where the light diffusers are. they have the ones with the small square holes about everywhere. Cut it to size, and keep it in place with suction cups.
Here is the one I made for my breeding tank.
I cut slots in 3/4" pvc,& slots for the suction cups.
This way all you have to do is slide it out to remove,& slide back in when needed.
My wife and I have one egg crate divider made up for each of the tanks...just in case. I have to agree with the DR. on this one Just... That certianly looks a lot better than the ones I made.