under gravel filters and suction cups work great for keepin fry away from bigger fish.. egg crate works great for keepin bigger fish away from other fish..
Egg crate makes an excellent sturdy divider.If you need smaller holes go to the craft store and ask for "plastic canvas" and just zip tie it to the canvas.
You can also buy some plastic peg board from the hardware store that would work great as a divider. Mike Dunagan bought some and used it to make a drip plate. I think it would work great for a divider. It would also make it a little harder for the fish to see each other.
Egg crate is good stuff, my only complaint about it is my fish attack through it and tear up their moutns. Covering it with thinner plasic should stop this though.