Reflective eggcrate

[slakker]

It doesn't rust, but can flake off. Some people don't like it and are concerned about it harming the fishes. You can spray urethane it if you want to be safe.

I found the easiest way to cut it is actually with side cutters. Just clip it, amazingly the plastic doesn't shatter or anything. I used a dremmel the first time and the melted plastic was a nusance.