Eggcrate is good for so many things, I think most of us have a least a partial sheet in our garages
You can get eggcrate at any of the big box home improvement joints. You just need to ask for a light diffuser as most people working in the stores will look at you with crossed eyes if you call it eggcrate
I used to use it when I had a tank with a plastic rim on it.. if your tank is eurobraced it makes it a little trickier to secure to the top of the tank.