preventing fish from jumping out?

There's no edging. All I did was cut what I needed from the 2x4 sheet I bought and placed it on the inside lip of the tank top. So it just sits there totally supported. If your tank has no inner lip I would just let it rest on top of the tank and to ensure it does not fall into the tank I would buy a couple of these soft plastic "nibs" that you use not to scratch the surface of a table. They come small enough to fit into the eggcrate holes. They are also stackable with one side having a self-adhesive strip.

If you look at my pic you'll see I've stacked one on top of another.

Very cheap $$ and very effective.

The hinges are from my glass top (which I don't use anymore). My tank is open top now and I have eggcrate in the front and in the back. I'll try and post another pic tonight of my present setup.