Thought I would share the cover I have been using to keep fish from jumping out of my tank. I have been using this mesh for tops for about 3 years and couldn't be happier. I used a glass top for several months, but don't like the glass because it accumulates condesation and salt spray which blocks light. I also had heat issues with the glass top.
The screen doesn't hold heat at all. After the halide was on all day, it is barely warm to the touch right under the bulb.
For those of you that use eggcrate, this top is also better because it allows more light to pass than eggcrate. With a metal halide, I got dark corners when I tried eggcrate. The light difference is hardly noticable with the clear mesh screen!
I have tried both 1/2 inch mesh and 1/4 inch mesh, both work very well. If you have real small fish, then the 1/4 is the way to go and doesn't compromise light intensity.
Here's photos of the top on the tank.